Also called the expanded roster, the 40-man is composed of all the players in a Major League club's organization who are signed to a major-league contract. These are the players who are able to be called up to the 25-man roster at any given time. Also on the 40-man are any players on the 15-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 "options" to be sent to the minors once on the 40-man 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 15-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 is the removal of a player from the 40-man, whereby the team has 10 days to trade the player, release him, or send him to the minors.
Read more about this topic: Major League Baseball Rosters
Other articles related to "roster":
... 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 40-man roster can play for the Major League team ...