"But I wonder how many of the categories or genres have been created in a way that is almost arbitrary" Well, yes, historically. Consider that back in the '60's virtually any LP that had a "black" (sorry folks, that's what they called them back then,) face on it was shoved into the R&B section of most record departments in the USA. I don't know how it was elsewhere. Even now I find traditional folk albums lumped in with the Country Music section (if they have any such albums at all!) I suppose it is the same with Rock Music. There are so many subdivisions of that, well, I hardly know where to begin!