Subgenres are an interesting sort of division.

You get them basically where people start saying "this book is like this one," and it ends up looking like a wall where someone threw paintballs at it -- big splotches and splatter, mostly around the splotches. But it isn't paintballs, it's just the effect of plotting individual works according to relevant traits. And while everyone agrees that the center of the splotch is that subgenre, it's where you draw the circle about it that produces quarrels.