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

    Modifies the time value specified with the /BEFORE or the
    /SINCE qualifier. The /BACKUP qualifier selects files according
    to the dates of their most recent backups. This qualifier
    is incompatible with the /CREATED, /EXPIRED, and /MODIFIED
    qualifiers, which also allow you to select files according to
    time attributes. If you do not specify any of these four time
    qualifiers, the default is the /CREATED qualifier.