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

       /EXTENSION=number-of-blocks

    Specifies the number of blocks to be added to the output file
    each time the file is extended. When you specify the /EXTENSION
    qualifier, the /NEW_VERSION qualifier is assumed and need not be
    typed on the command line. This qualifier is relevant only with
    the /NEW_VERSION qualifier.

    The extension value is applied only if a new file is actually
    created.