VMS Help SET, VOLUME, Qualifiers, /SUBSYSTEM  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /SUBSYSTEM
       /NOSUBSYSTEM

    Enables the processing of subsystem ACEs. Requires the SECURITY
    privilege. By default, the disk from which you boot has protected
    subsystems enabled but other disks do not. For further details on
    subsystems, refer to the OpenVMS Guide to System Security.