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

       /LABEL=(string[,...])

    Output Save-Set Qualifier

    Specifies the 1- to 6-character volume labels for the magnetic
    tapes to which the save set is written. If you do not specify
    the /LABEL qualifier, BACKUP uses the first six characters of the
    save-set name as the volume label of the first tape.

    If the save set continues to another tape, and you did not
    specify a volume label for the tape, BACKUP uses the first four
    characters of the previous tape's volume label followed by the
    volume number of the tape. For example, if the first tape in a
    save set is labeled AAAABB, the second tape in a sequence is
    labeled AAAA02, and the third tape is labeled AAAA03.

    You can specify the command qualifier /IGNORE=LABEL_PROCESSING
    to prevent BACKUP from verifying the volume label of the tape.
    You can also use the /EXACT_ORDER qualifier to specify the exact
    order of tape volume labels that you want to use in a BACKUP
    operation.