Now
THIS is a serious map of music.
I have approx. 120 genres for my main (metal) collection, with great majority being just combination of some styles (fe Death/Thrash Metal is not just some Death Metal and some Thrash Metal, but mixture of both) and variations of styles (Melodic Death, Progressive Death, Technical Death etc.). All these are based on characteristics of the music itself- chords, sound, composition etc.
In some single cases (bands), for some peculiar styles, I use special terms, like Riot Opera, Power a Capella, SID Metal, coined by the bands themselves or internet.
I use Vocals field to indicate untypical for a genre or mixed/multiple vocals, like growl, harsh, growl/shriek, harsh/clean/female operatic etc.
I use Instruments field to indicate any untypical instruments for a genre (fe. erhu, violin or piano).
All this tagging is done per album, as I listen to albums, so generally just one or two songs in an album with fe. specific vocals don't add anything in Vocals field - such situations I cover usually by per-track Featuring tag.