Each MLB club is permitted to reserve no more than 40 players on its MLB Reserve List, which is why the MLB Reserve List is also called the "40-man roster."

Each MLB club is required to roster at least 25 players but no more than 26 players (formerly a 24 player minimum and a 25 player maximum limit) on its MLB Active List beginning on MLB Opening Day up through 11:59 PM (Eastern) on August 31st, and then 28 players (formerly a 24 player minimum and a 40 player maximum limit) beginning on September 1st and extending through the last game of the MLB regular season. A club's MLB Active List roster must revert back to 26 players again for post-season play (LDS, LCS, and World Series). 
NOTE: In the event of an unusual or unavoidable circumstance (such as a multi-player trade, multiple trades made at about the same time, and/or multiple waiver claims awarded at about the same time, where several players have been acquired but have not yet reported), a club's MLB Active List roster can temporarily go below 25 players (beginning on MLB Opening Day and extending through August 31st) or below 28 (beginning on September 1st and extending up through the conclusion of the MLB regular season), but for no more than 48 hours.   

If a player is placed on the MLB CoViD-19 Related Injured List or on the Restricted List as the result of not being vaccinated, the player can be temporarily replaced on the MLB Active List roster by any player in the organization. When the player on the CoViD-19 IL or Restricted List is reinstated, the replacement player can be reassigned back to the minor league affiliate from which he was selected without requiring an Optional Assignment or Outright Assignment.
NOTE-1: The replacement player accrues one day of MLB Service Time for each day spent on the MLB Active List, but is not charged with an optional or outright assignment when sent back to the minor league affiliate.
NOTE-2: If a pitcher starts a game and pitches at least four innings and then is subsequently placed on the CoViD-19 Related Injured List or on the Restricted List for not being vaccinated, he cannot be replaced on the Active List roster for at least four days.

If MLB regular season games are scheduled prior to MLB Opening Day (for example, if MLB games are played in Japan, Mexico, Australia, etc), the MLB Commissioner will determine the Active List maximum roster limit for those games.   

Prior to the start of each MLB regular season a club must designate all players on its Opening Day Active List roster as either a "position-player," a "pitcher," or a "two-way player" (but only if the player already qualifies as a "two-way player") and once so designated the player's position designation cannot be changed for the balance of that season unless a "position-player" or a "pitcher" subsequently qualifies as a "two-way player" and is so designated during the course of the season.  

A maximum of 13 players on an MLB Active List can be designated as "pitchers" (14 "pitchers" maximum when rosters expand from 26 to 28 beginning on 9/1). 

For players who are recalled from an Optional Assignment or who have their contract selected and are added to an MLB Active List during the MLB regular season, the club must designate the player as either a "position player," a "pitcher, or a "two-way player" (if the player already qualifies as a "two-way player") when the player is placed on the club's MLB Active List roster.
NOTE: Since there is no limit on the number of "pitchers" who can be on a minor league club's active list roster, the "position-player," "pitcher," and "two-way player" designations are not used in the minor leagues.  

A player designated as a "position-player" can pitch in a game only if his club is winning by at least ten runs in the 9th inning, or losing by at least eight runs at any time during the game,  or regardless of the score if the game has gone to extra innings.  

With approval of the MLB Commissioner, a "two-way player" on an MLB Active List who has undergone elbow UCL reconstruction (so called "Tommy John surgery") can be temporarily assigned as a "pitcher" to a minor league club for rehabilitation purposes while remaining eligible to play in MLB games as a "position-player."  

A "position player" or a "pitcher" can be designated as a "two-way player" (and does not count against the maximum 13 pitchers allowed) if the player has thrown at least 20 IP during the course of the current MLB season or threw at least 20 IP in either of the two previous MLB seasons - AND - has started at least 20 games as a position player (including DH) and with at least three plate appearances in at least 20 of the games started as a position-player in the current MLB season or started at least 20 games as a position player (including DH) and with at least three plate appearances in at least 20 of the games started as a position-player in either of the two previous MLB seasons.   
NOTE: A "position-player" or a "pitcher" can qualify as a "two-way player" only if he has thrown at least 20 IP during the course of the current MLB season or threw at least 20 IP in the previous MLB seasons - AND - has started at least 20 games as a position player (including DH) and with at least three plate appearances in at least 20 of the games started as a position player in the current MLB season or started at least 20 games as a position player (including DH) and with at least three plate appearances in at least 20 of the games started as a position player in the previous MLB season.   

If a player who was designated a "two-way player" prior to the start of a season because he automatically qualified as a "two-way player" by virtue of meeting the "two-way player" requirements during a previous season but then was to fall below the IP and/or G/PA threshold by the end of the current season, he would not automatically qualify as a "two-way player" again at the start of the next season (he would count as a either a "position player" or a "pitcher" until he could re-establish himself as a  "two-way player").  

No more than 15 players (formerly no more than 16 players) on a club's MLB Reserve List (40-man roster) may be on Optional Assignment at any one time prior to September 1st, and no more than twelve players may be on Optional Assignment at any one time beginning on September 1st and extending through the conclusion of the MLB regular season.

"Position-players" who are optioned to the minors generally must remain on Optional Assignment for at least ten days and "pitchers" and a "two-way players" who are optioned to the minors generally must remain on Optional Assignment for at least 15 days. (A "position-player" optioned to the minors during Spring Training generally must remain on Optional Assignment for the first ten days of the MLB regular season, and a "pitcher" or a "two-way player" optioned to the minors during Spring Training generally must remain on Optional Assignment for the first 15 days of the MLB regular season).
EXCEPTIONS: The only exceptions are if the player is traded or claimed off waivers, recalled as the "27th man" for a doubleheader prior to September 1st (second game of the doubleheader only if two games were scheduled after a postponed or suspended game from the previous day) or other special circumstances, or is recalled to replace a player on the MLB Active List who is traded, claimed off waivers, or placed on the MLB Injured List, Bereavement/Family Medical Emergency List, or Paternity Leave List, or the Restricted List (but only pursuant to a suspension or administrative leave resulting from a violation or possible violation of the joint MLB-MLBPA JDV policy or a suspension resulting from a violation of the joint MLB-MLBPA JDPTP policy).

A club can recall a player ("position-player," "pitcher," or "two-way player") from a minor league Optional Assignment and place the player on its "Taxi Squad" for one day. If the player is not added to his club's MLB Active List by 3 PM (Eastern) the next day or at least three hours prior to the scheduled start of the next day's game (whichever is later), the player must be removed from the Taxi Squad and returned to his minor league assignment. A player on the Taxi Squad does not count against his club's MLB Active List or the minor league affiliate's active roster, and the player does not accrue MLB Service Time while on the Taxi Squad.
NOTE: The most common reasons to recall a player and place him on the Taxi Squad is when a club is considering whether to place a player on there Injured List (or other MLB inactive list) but has not yet decided, or if a trade is imminent but has not yet been consummated. 
CoViD-19 EXCEPTION: A club can carry up to five minor league players (one must be a catcher) on its MLB Taxi Squad who can be temporarily added to the club's MLB Active List roster to replace a player who is placed on the CoViD-19 IL.   

Prior to September 1st, a club can temporarily add one additional player (can be either a "position-player," a "pitcher," or a "two-way player") to its MLB Active List on any day where two games are scheduled (not including a day where the first game is the completion of a suspended game), as long as the second game was not scheduled as the result of a game that was postponed earlier in that series. If the postponed or suspended game was from earlier in the series, the additional player can be temporarily added to a club's MLB Active List for the second game only. Also, adding an additional player to a club's MLB Active List can be authorized by the MLB Commissioner for any game(s) involving unusual travel circumstances and/or any game(s) played in a non-MLB city or country.  

1. The extra player temporarily added to the club's MLB Active List must be on the club's MLB Reserve List (40-man roster) or has to be added to the club's 40-man roster that day.

2. The player temporarily added to the club's MLB Active List does not have to be a pitcher.

3. In the case of two games being scheduled on the same day where a player is temporarily added to a club's MLB Active List, the extra player cannot be switched between games.

4. A minor league player can be added to a club's Active List as the extra player even if he has not spent ten days (for a "position-player") or 15 days (for a "pitcher" or a "two-way player") on Optional Assignment (or Outright Assignment) prior to being added.
   
7. A player added to a club's MLB Active List as an extra player can remain on the Active List and a different player can be dropped the next day, as long as the extra player was not called up from the minors prior to spending at least ten days (for a "position-player") or 15 days (for a "pitcher" or for a "two-way player") on Optional or Outright Assignment. However, a player called up as the extra player prior to spending at least ten days (for a "position-player") or 15 days (for a "pitcher" or for a "two-way player") on Optional or Outright Assignment could remain on the club's MLB Active List roster if the player replaces a player who is placed on an injured list or other MLB inactive list the next day.    

8. If a player is temporarily recalled as an extra player and then is optioned or outrighted back to the minors the next day, the "10-day rule" clock (for "position-players") or "15-day rule clock" (for "pitchers" and for "two-way players") that prohibits a player from being recalled until he has spent at least ten days (for "position-players") or 15 days (for "pitchers" or for "two-way players") on Optional or Outright Assignment (unless he is replacing a player who is placed on an injured list or other MLB inactive list) does not start over again. However many days toward ten (for a "position-player") or 15 (for a" pitcher" or for a "two-way player") that the player spent on Optional or Outright Assignment prior to being recalled as an extra man counts toward the ten days (for a "position-player") or 15 days (for a "pitcher" or for a "two-way player").    

A player temporarily added to a club's MLB Active List for a doubleheader or other circumstances accrues one day of MLB Service Time.

Only players on an MLB Reserve List (40-man roster) can be placed on an MLB Active List.

A free-agent with prior MLB and/or minor league service who signs a Major League contract must be added to his club's MLB Reserve List (40-man roster) when he is added to an MLB Active List or when the contract is filed with the MLB Commissioner (whichever comes first). Contracts must be filed with the MLB Commissioner within 20 days of signing.

A club must add a player to its MLB Reserve List (40-man roster) immediately if the player is acquired as the result of a waiver claim, or if a player on another club's 40-man roster is acquired in a trade, or if a player is selected in the Major League Phase of the Rule 5 Draft. 
NOTE: A player has up to 72 hours to report after being assigned from one MLB club to another. The player counts against the club's MLB Reserve List (40-man roster) as soon as the player is claimed off waivers or as soon as a trade is filed with the MLB Commissioner, but the player does not count against the club's MLB Active List until he physically reports to the club and is in uniform. 

A player on an MLB 60-day Injured List who is eligible to be an Article XX-B MLB free-agent must be reinstated no later than 9 AM (Eastern) on the day after the final game of the World Series, and all other players on an MLB 60-day Injured List as well as any player on a minor league 60-day IL must be reinstated no later than 5 PM (Eastern) on the 5th day after the final game of the World Series, even if the player did not spend 60 days on the IL.  

A player on an MLB Reserve List (40-man roster) who is given his Outright Release during the period of time extending from September 1st through the day prior to MLB Opening Day cannot be re-signed and added back to the MLB Reserve List (40-man roster) of the club that released the player until May 15th, and a player on an MLB 40-man roster who is given his Outright Release during the period of time extending from MLB Opening Day through August 31st cannot be re-signed and added back to the MLB Active List of the club that released the player for at least 30 days.   

A player can be placed on his club's Military List within 15 days of the date he is scheduled to report for duty, but only if the player is no longer physically with the club. A player on the Military List must be reinstated within 15 days after being released from service but a player on the Military List can spend up to 15 days on a minor league "conditioning" assignment prior to being reinstated to an MLB or minor league reserve list, and a player who is reinstated from the Military List after the MLB Rule 5 Draft but prior to MLB Opening Day does not count against his club's Reserve List until MLB Opening Day, or up to 15 days after MLB Opening Day if the player is on a minor league "conditioning" assignment during that period of time.   

An MLB player who is on the Voluntary Retired List or on the Ineligible List cannot be reinstated from August 1st through October 31st, and a minor league player on the Voluntary Retired List or on the Ineligible List cannot be reinstated from August 1st through the end of his club's season (including post-season).     

An MLB player who is on the Disqualified List or on the Restricted List for any reason other than extended Bereavement/Family Medical Emergency Leave or a suspension for violation of the Joint Drug Prevention and Treatment Program (ban on the use of a prohibited substance) or a suspension or leave of absence related to a violation or possible violation of the Joint Domestic Violence, Sexual Assault, and Child Abuse policy cannot be reinstated from August 1st through October 31st without consent of the MLB Commissioner, and a minor league player on the Disqualified List or on the Restricted List for any reason other than a suspension for a violation of the Joint Drug Prevention and Treatment Program cannot be reinstated from August 1st through the end of his club's season (including post-season) without consent of the MLB Commissioner. 

A player on the Ineligible List must be added back to his club's reserve list within 30 days after reinstatement if he is reinstated during the period of time beginning on the mandatory Spring Training reporting date and extending through July 31st or by Opening Day if he is reinstated during the off-season, and a player on the Restricted List or Disqualified List must be added back to his club's Reserve List 30 days after reinstatement or when he is placed on the club's Active List (whichever comes first) if the player is reinstated during the period of time beginning on the mandatory Spring Training reporting date and extending through the last day of the regular season or within 14 days after reinstatement if the player is reinstated during the off-season, but if the player is assigned to another club while on the Restricted List he must be placed on that club's Reserve List immediately after reinstatement.

A player on the Voluntary Retired List must spend at least 60 days of the regular season on the list before he can apply for reinstatement. If the player is placed on the Voluntary Retired List within 60 days of the conclusion of the regular season, the balance of the 60 days would carry-over into the next season.

In addition, an MLB club generally has the option to add any player who is on the reserve list of one of its minor league affiliates to its MLB Reserve List (40-man roster) at any time. 

The exceptions are:

1. A player on a minor league reserve list who is eligible for selection in the MLB Rule 5 Draft cannot have his contract selected and be added to an MLB Reserve List (40-man roster) during the period of time beginning with the filing of MLB and minor league reserve lists in November through the conclusion of the Rule 5 Draft.

2. A player on a minor league reserve list cannot have his contract selected and be added to the MLB Reserve List (40-man roster)  during the period of time beginning on the first official day of MLB Spring Training through March 14th. 

3. A "position-player" who was sent outright to the minors generally cannot have his contract selected and be added back to his club's MLB Reserve List (40-man roster) for at least ten days and a "pitcher" or a "two-way player" who was sent outright to the minors generally cannot have his contract selected and be added back to his club's MLB Reserve List (40-man roster) for at least 15 days. (A "position-player" outrighted to the minors during Spring Training generally cannot have his contract selected and be added back to his club's MLB 40-man roster for the first ten days of the MLB regular season and a "pitcher" or a "two-way player" who is outrighted to the minors during Spring Training generally cannot have his contract selected and be added back to his club's MLB 40-man roster for the first 15 days of the MLB regular season).
EXCEPTIONS: A "position-player," "pitcher," or "two-way player" is temporarily called-up as the extra player for a doubleheader (second game of the doubleheader only if two games were scheduled after a postponed game from earlier in the series) or other circumstances, or is called-up to replace a player on the MLB Active List who is traded, claimed off waivers, or placed on the MLB Injured List, Bereavement/Family Medical Emergency List, Paternity Leave List, or Restricted List (but only pursuant to a suspension or administrative leave resulting from a violation or possible violation of the joint MLB-MLBPA JDV policy or a suspension resulting from a violation of the joint MLB-MLBPA JDPTP policy).
NOTE: A player temporarily called up as an extra man can remain on the Active List and a different player can be dropped the next day, as long as the extra man was not called up from the minors prior to spending at least ten days ("position-player") or 15 days ("pitcher" or "two-way player") on Optional or Outright Assignment. However, a player temporarily called up as an extra man prior to spending at least ten days ("position-player") or 15 days ("pitcher" or "two-way player") on Optional or Outright Assignment could remain on the MLB Active List if the player replaces a player who is traded, claimed off waivers, or placed on the MLB Injured List, Bereavement/Family Medical Emergency List, Paternity Leave List, or Restricted List (but only pursuant to a suspension or administrative leave resulting from a violation or possible violation of the joint MLB-MLBPA JDV policy or a suspension resulting from a violation of the joint MLB-MLBPA JDPTP policy).   

4. If a minor league player on Outright Assignment has his contract selected and is called-up as the temporary extra man for a doubleheader or other circumstances and then is either optioned or outrighted back to the minors the next day, the "10-day rule" clock (for "position players") or "15-day rule" clock (for "pitchers" and for "two-way players") that prohibits a player from being called-up after being outrighted until he has spent at least ten days ("position-player") or 15 days ("pitcher" or "two-way player") on Outright Assignment does not reset or start over again. However many days toward ten (for a "position-player") or 15 (for a "pitcher" or for a "two-way player") that the player spent on Outright Assignment prior to having his contract selected and being temporarily called-up as the extra man counts toward the ten days (for a "position-player") or 15 days (for a "pitcher" or for a "two-way player") the player must spend on Outright and/or Optional Assignment. 

5. An unsigned minor league player who is eligible to be an MLB Rule 9 (formerly MLB Rule 55) minor league free-agent (player was previously released in his career and/or player has spent all or part of at least seven separate seasons on a minor league active list or disabled list) cannot have his contract selected and be added to his club's MLB Reserve List (40-man roster) any later than 5 PM (Eastern) on the 5th day after the final game of the World Series (or 5 PM Eastern on October 15th if the World Series is canceled). 

After the conclusion of the MLB season, the status of players on a club's MLB Reserve List (40-man roster) who are under contract for the following season is considered to be both "reserved" and "signed." The status of the other players on the 40-man roster are considered to be both "reserved" and "unsigned."

Unsigned players on an MLB club's Reserve List (40-man roster) would include any player who was added to an MLB Reserve List (40-man roster) after the conclusion of the MLB regular season, any player who can have his contract unilaterally renewed by his club, any player who is eligible for salary arbitration, and any player eligible to be an MLB free-agent per Article XX-B of the CBA.

Technically, a baseball trade is called a Trade Assignment (similar in kind to an Optional Assignment or Outright Assignment). 

Generally, an MLB club can trade a player on its MLB Reserve List (40-man roster) and minor league reserve lists at any time.  However, there are a few restrictions:

1. A trade involving a player signed to an MLB contract (including any player on an MLB Reserve List, MLB 60-day Injured List, or MLB Restricted List, and/or any player who was outrighted to the minors after signing an MLB contract for that season) is prohibited beginning at 4 PM (Eastern) on July 31st (or 4 PM Eastern on Friday July 30th if July 31st falls on a Saturday, or 4 PM Eastern on Monday August 1st if July 31st falls on a Sunday) and extending until the day after the final game of the World Series.  
NOTE-1: Beginning with the 2022 season, the MLB Commissioner has the authority to establish the MLB Trade Deadline on any day over the seven-day period July 28 - August 3. However, the MLB Trade Deadline cannot be on the same day as the MLB Rule 4 Draft (First Year Player Draft) signing deadline. 
NOTE-2: Trade Assignment Waivers (which permitted the trading of players on MLB reserve lists after the July 31st deadline up until 12 PM Eastern on the 7th day prior to the originally-scheduled conclusion of the MLB regular season) were eliminated starting with the 2019 season. 

PLAYERS OUTRIGHTED TO MINORS AFTER SIGNING 2023 MLB CONTRACT: (updated 9-12-2023)
David Bote, INF
Chris Clarke, RHP
Shane Greene, RHP
Edwin Rios, 1B 

2. A trade involving a player on a minor league reserve list who is not an outrighted player is prohibited beginning at 12 PM (Eastern) on the 7th day prior to the originally-scheduled conclusion of the MLB regular season through the last day of the MLB regular season (including a day on which a regular season game is played after the originally-scheduled conclusion of the MLB regular season).   

3. A player who signs after being selected in the MLB Rule 4 Draft (First-Year Player Draft) cannot be traded until after the conclusion of the World Series (no earlier than 9 AM on the day after the final game of the World Series), or for 90 days if the player signs after the World Series.   

4. A minor league player eligible for selection in the Rule 5 Draft cannot be added to an MLB Reserve List (40-man roster), traded to another organization, or transferred from one minor league reserve list to another within the same organization, beginning with the filing of MLB & minor league reserve lists in November and extending through the completion of the Rule 5 Draft.

5. A player cannot be traded while he is on waivers.

6. A player on an MLB Reserve List (40-man roster) who has a contractual "no trade" right can waive this right if he so chooses.

PLAYERS ON CUBS MLB RESERVE LIST WITH CONTRACTUAL "NO-TRADE" RIGHT: (updated 5-25-2023)
Ian Happ, OF (full)
Seiya Suzuki, OF (full) 
Dansby Swanson, INF (full) 
Jameson Taillon, RHP (partial)

7. A player on an MLB Reserve List (40-man roster) who has accrued at least ten years of MLB Service Time with at least the last five years of MLB Service Time accrued with the same club (a so-called "10/5" player) has an automatic "no trade" right for as long as he remains with that club. The player can waive this right if he so chooses.

"10/5" PLAYERS ON CUBS MLB RESERVE LIST: (updated 11-14-2022)
NONE AT THIS TIME

8. An Article XX-B MLB free-agent who signs a Major League contract after 11:59 PM (Eastern) on the 5th day after the final game of the World Series has an automatic "no trade" right through June 15th. The player can waive this right, but if he does he can be traded only for cash and/or player contracts with a maximum aggregate value of $50,000. N
NOTE: An Article XX-B MLB free-agent who signs a minor league contract after 11:59 PM (Eastern) on the 5th day after the final game of the World Series does NOT receive an automatic "no trade" right, even if the player is later added to the club's MLB Reserve List (40-man roster).  

POST-2023 ARTICLE XX-B MLB PLAYERS ON CUBS MLB RESERVE LIST: (updated 6-16-2023)
TBD

9. If a "Player to Be Named Later" (PTBNL) is part of a trade, the PTBNL cannot be on an MLB Active List at any time beginning when the trade is executed (filed with the MLB office) up until when the trade is completed. A PTBNL can be either a specific player or the PTBNL can be selected from a list of players or a class of players as determined by the clubs (in writing) when the trade is executed. 
NOTE: A player signed after being selected in the MLB Rule 4 Draft (First-Year Player Draft) cannot be a PTBNL in any trade made prior to the conclusion of the World Series. Clubs have six months to agree on a PTBNL, but the clubs can agree (when the trade is executed) on a deadline that is less than six months. A cash payment (typically $50,000 for trades involving players on the 40-man roster) can be substituted for a PTBNL if no agreement can be reached within six months, but the alternative cash payment must be agreeable to both clubs and stated in writing when the trade is executed. 

10. A player on an MLB or minor league injured list can be traded, even if the player is not eligible to be reinstated and/or healthy enough to play.
NOTE: As far as a player having to spend a certain number of days on an IL before he is eligible to be reinstated is concerned, in the case of a trade, time already spent on an injured list is carried-over to the player's new club. 

11. If a player is acquired in a trade and is optioned to the minors by his new club within 24 hours, the player can be recalled from the Optional Assignment at any time without restriction (so that the "ten-day rule" for position players and the "15-day rule" for pitchers & two-way players as it pertains to the minimum number of days a player must spend on optional assignment before he can be recalled would not apply).