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

       /VOLUME=n

    Command Qualifier

    Indicates that a specific disk volume in a disk volume set is to
    be processed. The /VOLUME qualifier is valid only when used with
    the /IMAGE qualifier. Using this qualifier allows you to perform
    BACKUP operations on a disk volume set when you have only one
    disk drive with which to perform BACKUP operations.