The name of this XCMD is 'ButtonRgn'. However, one of the parameters passed to it tells it which of 3 subroutines within it to call.
The general form of the XCMD call is:
ButtonRgn "<handle>","<routine name>", other parameters
where <handle> is the name of a GLOBAL variable used to hold a memory handle to the regions describing the buttons, and <routine name> is one of the routines described previously.