This is going to be up to the director, but I think you basically don't show anything that can't be seen in a tiny string bikini. So there is a strong suggestion of nudity (because there are no strings or bikini) by the setting and pose, but no frames showing nipples or genitals or pubic hair.
Note the actor (male or female) may be nude while filming; or wearing stick on coverings; and this discreteness is accomplished by the director setting poses and then in editing judiciously cutting frames (or switching cameras, or ending scenes) when the prohibited body parts (or the coverings) appear.
In Titanic a nipple is shown, briefly (20 sec?), a naked butt, and there is a drawing of full frontal nudity, not the real thing. It gets a PG-13 rating because the nudity is brief and is not part of a sex scene; despite sexual tension in the scene, it was an art scene (the girl was posing for a drawing). (However, some believe that rating was allowed due to the stature of the director (James Cameron) and the budget of the film.)
Intercourse and orgasm are implied with zero nudity, by having it occur in a car with steamy windows; basically any kind of body concealment will suffice (a blanket, shower curtain, soapy water, or just something in the way of seeing the prohibited body part, like an arm or furniture). This is a reason many sex scenes are filmed with the actors under covers (unrealistic in my experience), or are "spontaneous" in full clothing, it is easy to conceal intercourse under a skirt (and to simulate it).