<P> The expanded roster includes all players who are eligible for call - up to the active roster at any given time . Also on the 40 - man are any players on the 10 - day disabled list and minor league players who are signed to a major - league contract but are on an "optional assignment" to the minors . (Each player has three "option years" to be sent to the minors once on the expanded roster before they must be placed on waivers to be sent there .) Players who were on the 40 - man but are placed on the 60 - day disabled list are taken off the 40 - man until the time on the DL is over . The same applies to players who are suspended . Because players on the 60 - day DL are taken off the 40 - man with no risk of losing the player, MLB teams often transfer injured players from the 10 - day DL to the 60 - day DL so that they can add another player to the 40 - man without having to designate a player for assignment . Designating for assignment ("DFA") is the removal of a player from the expanded roster, whereby the team has 10 days to trade the player, release him, or send him to the minors . </P> <P> On September 1, the Major League team's roster expands from the 25 - man active roster to the entire 40 - man roster . At this point, any player on the expanded roster on August 31 may play for the Major League team . </P> <P> September call - ups are players from the minors who are playing in September to get Major League experience and, especially for teams in contention, to provide reinforcements down the stretch . Note, however, that once the roster expands, in practice teams will utilize a number of September call - ups (typically up to six), but seldom if ever the full 15 extra players that the rules allow . </P> <P> A postseason roster takes effect only if a team clinches a playoff berth . Players who are part of the team's final roster at the end of the regular season are eligible to participate in the postseason . Any player who has been traded from a different team, spent time in the Minor Leagues, or signed later in the season with the team (no later than August 31) is eligible to participate in the postseason . A postseason roster is allowed up to 25 active players . Other players who are not on the 25 - man active roster will be assigned to the postseason secondary squad . Players who are on the disabled list or any other non-active transaction by the end of the regular season will have their transactions passed on in the postseason . Rosters for a series are set at the beginning of the series and no changes to the 25 - man active roster are allowed except when a player is moved to the disabled list or any other inactive transaction . If a player is moved to the disabled list or another inactive transaction during a series, he then becomes ineligible to be returned to the 25 - man active roster for the remainder of the series as well as the next series if applicable . If any player goes on any inactive transaction, any player from the 40 - man roster can be promoted to the 25 - man active roster for the remainder of the series if applicable . </P>

When do mlb playoff rosters have to be set