A user must belong to at least one group and can be connected to additional groups. When a user connects to the system (that is, logs on to a TSO/E session), one of the groups is selected as the user's current group. For a user to be able to request kernel services and invoke the shell, the user's current RACF® group must have a z/OS UNIX group ID (GID) assigned to it. All groups that a user belongs to should be assigned an OMVS GID. Also, the user's default group must have a GID assigned for POSIX standards conformance.