In the Army on the Unit Manning Document you can fill a position that is billeted one rank higher or two ranks lower than your current rank.

For instance, each unit is allotted so many personnel at each rank. They can have so many privates, so many specialists, so many sergeants. But in some situations a specialist (E-4) can fill a sergeant (E-5) position; or a sergeant (E-5) could be placed in a private first class (E-5) position.

Usually those types of slotting moves are temporary.

I'm guessing the questioner was thinking of that. And in the Army you do try to make sure that everyone has a good idea of what the guy above them is supposed to be doing. Platoon Sergeants should be able to immediately step in and fill the role of First Sergeant. Section/Squad Leaders the same for the Platoon Sergeant. Team Leaders for the Squad Leader role.

It was a weird question.