VMS Help EXCHANGE, 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. This qualifier is valid for
    Files-11 output files only.

    EXCHANGE determines the default extension according to the
    following hierarchy:

    1. An explicit value specified on the /EXTENSION qualifier

    2. The current process default extension value set by the command
       SET RMS_DEFAULT

    3. The current system default extension value set at system
       generation or with the SET RMS_DEFAULT/SYSTEM command

    Use the /EXTENSION qualifier to set an extension quantity with
    magnetic tape input; EXCHANGE preallocates a file of the correct
    size when the input is on a directory-structured-device.