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

       /EXTENSION=n

    Specifies the number of blocks to be added to the output
    file each time the file is extended. If you do not specify
    the /EXTENSION qualifier, the extension attribute of the
    corresponding input file determines the default extension
    attribute of the output file.