From Wikipedia:
Concussion List
Starting with the 2011 season, Major League Baseball has instituted a new disabled list, a 7-day list specifically for concussions and brain damage. The idea is to prevent long-term brain damage which may take up to 7 days instead of using the full 15 days by current standards. MLB has ensured this is only for concussions, and will take steps to avoid abuse of the system.[3] If a player is not activated from the concussion DL after those 7 days have passed, he is automatically transferred to the 15-day DL.
I don't think it gets used often...