VMS Help TCPIP Services, Programming Interfaces, RPC XDR Routines, xdr_union, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 xdrs
    A pointer to an XDR stream handle created by one of the XDR
    stream handle creation routines.
 dscmp
    A pointer to the union's discriminant.
 unp
    A pointer to the union's data.
 choices
    A pointer to an array of xdr_discrim structures. Each structure
    contains an ordered pair of [value,proc]. The final structure in
    the array is denoted by a pointer with the value NULL.
 default
    The address of the default XDR routine to call if the dscmp
    argument is not found in the choices array.