You'll note that all functions begin with the letters MIDI. To avoid conflicts with MIDI Rexx function names, do not name any of the functions in your REXX script starting with those 4 letters (neither upper nor lower case).
If any of the functions are called before a MIDI file is created/loaded, then they usually 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 the ERROR condition if you trap that. For example, if you tell MIDIGetEvent() to return the next event of a certain type, and there is no more of that type, then MIDIGetEvent() 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 a MIDI Rexx function where appropriate.
The sections of this book leading up to the list of functions describe how to do various tasks with the MIDI Rexx functions. So, they are overviews (ie, tutorials) of how to use MIDI Rexx in various ways, whereas the list of functions are more of a reference complete with incidental notes/details.