VMS Help BACKUP Command, Qualifiers, /RELEASE_TAPE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Command Qualifier

    Dismounts and unloads a tape after a BACKUP save operation writes
    a save set to the tape (and optionally verifies the saveset
    information on that tape). By using the /RELEASE_TAPE qualifier
    in conjunction with either the /DELETE or /RECORD qualifiers, you
    can make a tape drive available for other operations before the
    BACKUP command completes.

    The tape remains allocated until you enter the DEALLOCATE
    command.