Under Unix, every running program has an associated user id, which normally corresponds to the user that started the process. This id largely dictates the actions permitted to the program. No outside process can change the user id of a running program. A Hurd process, on the other hand, runs under a "set" of user ids, which can contain multiple ids, one, or none. A sufficiently privileged process can add and remove ids to another process. For example, there is a password server that will hand out ids in return for a correct login password.