I'm no lawyer, so technically, I can't specify exactly what the legal difference would be.
As I am sure you know, there is statuatory rape.
Having consentual sex with a partner who is legally underage but was willing.
Assault and battery combined with sex would, to me, imply physical injury to the person.
Another example, is where a female is drugged, and then raped. She would likely suffer no physical injury, didn't have to be held down, so it would just be Rape.
In this case, it is alledged that the female was consenting to normal sex, but that the male in question allegedly wanted deviant sex, and the female didn't.
Also alledged is that she resisted and was physically abused (beaten?). That is SEXUAL ASSAULT and BATTERY.
Thank you for your reply and allowing me an opportunity to explain my understanding of this difficult subject.