The setuid function can be used with POSIX style identifiers
    enabled or disabled.

    POSIX style IDs are supported on OpenVMS Version 7.3-2 and
    higher.

    With POSIX IDs disabled (the default), the setuid function is
    implemented for program portability and serves no function. It
    returns 0 (to indicate success).

    With POSIX style IDs enabled:

    o  If the process has the IMPERSONATE privilege, the setuid
       function sets the real user ID, effective user ID, and the
       saved set-user-ID to uid.

    o  If the process does not have appropriate privileges but uid
       is equal to the real user ID or to the saved set-user-ID, then
       the setuid function sets the effective user ID to uid. The
       real user ID and saved set-user-ID remain unchanged.

    See also getuid to know how UIC is represented.