You could divide R&B into categories, genres, and subgenres ad infinitum. Doesn't make it so. Always remember it's the music INDUSTRY. As in - the bottom line is money. All these categories are just that. Ohhh, now we can repackage a bunch of songs and call it whatever and make more scratch from royalties WE ALREADY OWN.
Personally, I think all of it is bullshit. I'd recognize maybe 5 subgenres, and that's about it.