Out of the Park Developments Online Manuals
 
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Expanded Rosters
The active roster remains at the established limit for most of the season. However, in most leagues, a date is set after which the active roster size grows to a larger size (most typically, the size of the secondary roster). These are known as expanded rosters. The roster expansion date is typically near the end of the regular season, and often after the end of the minor league season. Roster expansion gives teams an opportunity to bring up young talent from the minors who otherwise wouldn't see much playing time. Since this typically happens in September in the real world, this is often referred to as "September callups." Rosters return to normal size on the first day of the playoffs. In OOTP, the active roster also expands during Spring Training, but this does not follow the same rules we're talking about here.

Expanded Roster Settings
As with most of these settings, settings for roster expansion are customizable in the Roster Rules section of the Rules page during game creation, or after game creation by navigating to OOTP Menu | Game Setup | League Setup tab | Rules tab. You can set the size of the expanded rosters from 15 to 70, disable expanded rosters entirely, and set the roster expansion date.

Things to Consider with Expanded Rosters
A GM should be careful, though, about how he uses expanded rosters. You may feel inclined to bring up all of your hot young talent. However, remember that as soon as you place a minor leaguer on the active roster, he receives a major league contract and begins to accrue major league service time, setting him on the path to free agency. So, it's often best to bring players up only if you need them to rest starters for the playoffs, or if you foresee them being regular major leaguers in the near future.