You'll note that all functions begin with the letters MidiIo. To avoid conflicts with RxMidiIO function names, do not name any of the functions in your REXX script starting with those 6 letters (neither upper nor lower case).
If certain functions are called before a MIDI port is created/loaded, then they may perform no work and raise a REXX SYNTAX condition. Failure to supply required arguments (or supplying invalid arguments) also raises a SYNTAX condition. Other errors do not raise a SYNTAX condition, but instead return an appropriate error indication to your script, or may even raise some condition if you choose that. For example, if you tell MidiIoOpenPort() to open a specific port, and there is an error, then MidiIoOpenPort() returns an error message. It's up to your script to check returned values for any errors in performing an operation. Always do error checking upon the return from an RxMidiIO function where appropriate.
The sections of this book leading up to the list of functions describe how to do various tasks with the RxMidiIO functions. So, they are overviews (ie, tutorials) of how to use RxMidiIO in various ways, whereas the list of functions are more of a reference complete with incidental notes/details.