Players are placed on either the 10-day (prior to the 2017 season, 15-day) or the 60-day disabled list, usually depending on the severity and/or recovery time of the injury. A player may be shifted from the 10-day to the 60-day DL at any time, but not vice versa. The player may not rejoin the team until 10 or 60 days has elapsed; however, a player's time on the DL may exceed the specified number of days, and, further, if a player is transferred to the 60-day DL after August 1, he may not return to the active roster that season. The rule about rejoining the team only applies to active duty. Players are permitted to stay with the team and attend games, though players may leave the team for short term minor league rehabilitation assignments to prepare for their return to the active roster.
The 10-day DL does not count the player on the active roster (comprising the 25-man roster until September 1), whereas the 60-day DL does not require the player to be counted on either the team's active roster or its 40-man roster; however, a team's 40-man roster must be full in order for the option of a placement on the 60-day disabled list to be available.
Placing a player on the disabled list opens a spot on the active roster. Another player from the minor leagues, free agent pool, a traded player, or a recovered player coming off the disabled list may be used to fill this spot. This allows a team to avoid being penalized because it avoids the disadvantage of playing with a reduced roster.
Starting with the 2011 season, Major League Baseball instituted a new disabled list, a 7-day list specifically for concussions. The idea is to prevent long-term brain damage which may take up to 7 days by current standards. If a player is not activated from the concussion DL after those 7 days have passed, he is automatically transferred to the 10-day DL.