VMS Help SHOW, SECURITY, 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 (rather than by the creation,
    expiration, or modification date). By default, SHOW SECURITY
    selects files according to their creation date.