billet

"A personnel position or assignment that may be filled by one person." [JCP1] (Compare: principal, role, user.)

Tutorial: In an organization, a "billet" is a populational position, of which there is exactly one instance; but a "role" is functional position, of which there can be multiple instances. System entities are in one-to-one relationships with their billets, but may be in many-to-one and one-to-many relationships with their roles.