It seems to be
a system disk
used to release
the treatment room
MO Disk location

System Disk location

System Disk can be found by Jill Valentine in the Treatment control room. This item is necessary for entering the Treatment room.

Checking the door of the Treatment room, Jill can read: "The door is solidly locked.". The panel next to the door reads: "Please insert a System Disk to unlock."

It is noticeable, that after using the disk near the room panel, Jill will enter the Treatment Room on the cutscene. After using the disk, this item will disappear.