Mullions are used on a pair of doors to add security.
Fixed mullions become a part of the frame between two doors. This type of mullion will actually separate the doors which causes the doors to become individual doors within the frame. Selecting a fixed mullion will either increase the rough opening width or reduce the door width sizes. As by the name, once in place they can not be removed.
Removable mullions are a piece of hardware that is only used at a pair of doors in which the doors stop against the mullion and the mullion effectively seals the gap between them. The mullion can be removed with a keyed cylinder can be installed so that a cut key is required.