Good Morning JK.......
Normally a person who is carried on a units books for pay and rations but does not have a specific job....... Can be while recovering from wounds, on training or just joined unit but not being used at the time......
On line definition:
Supernumerary
Supernumerary is an adjective which means "exceeding the usual number". When used as a noun, "supernumerary" means a temporary employee, additional society member, or extra manpower, usually in a function which has a temporary contract. Its counterpart, "numerary", is a civil designation for persons who are incorporated in a fixed or permanent way to a society or group, meaning a regular member of the working staff; permanent staff or member.
Mike