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

    Output File Qualifier

    Writes over an existing file when an identically-named file is
    encountered during the restore operation. If the identically-
    named files are not the same size, the file is extended (when the
    file being restored is larger) or retains its allocation (when
    the file being restored is smaller).

    By default, BACKUP does not restore a file with the same file
    name, file type, and version number as a file that already exists
    in a directory. Rather, BACKUP issues an error message and does
    not restore the file. The /NEW_VERSION and /REPLACE qualifiers
    also override this default, but produce different results than
    the /OVERLAY qualifier.