Angelina i admit that i am sometimes a bit small-minded, but i, for example, would imagine a BDSM relationship that involves one partner being led on a dog leash by the other partner all day long, even in public. that is certainly wrong, but i would associate it with that.
As I said, it is only a description. A suitable example would be:
Class: Car
Including subclass: sports car, station car, bus, truck.
It is only a description to differentiate it and not to rank it.