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

    Modifies the time value provided with the /BEFORE or the /SINCE
    qualifier. The /BACKUP qualifier selects files according to the
    date of their most recent backup (rather than by the creation,
    expiration, or modification date). By default, SET SECURITY
    selects files according to their creation date.