There isn't a manual but there is this forum

Category matches the FreeDB category ie rock, classical and folk etc. There are only eight or nine possibilities for this. Genre expanded that with a subset of SubGenre but both entries coming off the same list (maintainable through preferences).

Preferences is also where you'll find the reference format - in the example you gave you have category + a number which increments on each addition but I have set it to take just the number in my database.