The command language interface (CLI) routines process command
    strings using information from a command table. A command table
    contains command definitions that describe the allowable formats
    for commands. To create or modify a command table, you must
    write a command definition file and then process this file with
    the Command Definition Utility (the SET COMMAND command). For
    information about how to use the Command Definition Utility,
    see the VSI OpenVMS Command Definition, Librarian, and Message
    Utilities Manual.