VMS Help RTL Routines, LIB$, LIB$RESET_VM_ZONE, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 zone-id

    OpenVMS usage:identifier
    type:         longword (unsigned)
    access:       read only
    mechanism:    by reference

    Zone identifier. The zone-id is the address of a longword that
    contains the identifier of a zone created by a previous call to
    LIB$CREATE_VM_ZONE or LIB$CREATE_USER_VM_ZONE.