/sys$common/syshlp/helplib.hlb COPY, Qualifiers, /OVERLAY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /OVERLAY
       /NOOVERLAY (default)

    Requests that data in the input file be copied into the existing
    specified file, overlaying the existing data, rather than
    allocating new space for the file. The physical location of
    the file on disk does not change; however, for RMS indexed and
    relative files, if the output file has fewer blocks allocated
    than the input file, the copy fails giving an RMS-E-EOF error.

    The /OVERLAY qualifier is ignored if the output file is written
    to a non-file-structured device.