/sys$common/syshlp/helplib.hlb RPCGEN, Qualifiers, /DEFINE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /DEFINE = (name[=value][,....])

    Optional.
    DIGITAL UNIX equivalent: -D
    Default: No definitions.

    Defines one or more symbol names. Equivalent to one or more
    #define directives. Names are defined as they appear in the
    argument to the qualifier. For example, /DEFINE=TEST=1 creates
    the line #define TEST=1 in the output files. If you omit the
    value, RPCGEN defines the name with the value 1.