I have tried to achieve my goal, and think I've understood the principle! But I need some advice to continue, please.
I have manged to remove the original Genre Entries, except for the two default ones of . and , which I think will be required when adding future mp3s - the former for certain.
I have seen - and clicked the box to allow only the genre entries that I make, to remain visible.
I can see I think) where the original entries for the Genre field are stored, and have no problem just inserting one of my own custom genre entries. But I am unsure of the protocol for entering the 25 or so remaining entries, and it would seem that I have to add one at a time.
In the file that, I think, housed/houses the Genre-specific information, all I saw was Audiobook=, and visibly it looks like a Text document.
Is this the place where wy own curated Genre list would reside, and, while I want it to appear something like the following:
etc, I would assume that there would be a special way of entering the entries, so that they appear as a dropdown list as before, but my list, rather than the default.
At the moment it's no hardship, because each mp3 has 'remembered' its Genre setting for each of its constituent tracks, but when I come to add a new mp3, instead of being able to selent my Genre-marking system, rather than have to look it up, because, while it seems complicated, I understand its significance.
Hopefully with a little more help, I can get there without screwing up!