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

       /EXTENSION=n

    Qualifier type:        Output file qualifier

    Supported file types:  Local

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

    Default value:         The default extension for the local system
                           - usually 3 (nonstandard default)