Actually, considering that the site in question actively markets products and derivites of ....software which, above and beyond has , at its core, complete nudity as a default,  (and encourages a gallery to help selI same), would find it irresponsible not to have its own definition of "nudity".
Of course, they didn't really need that before, but, you know, once they go and make new rules, it's time for some definitions, I'd say.

Oh, and why couldn't you put in the terms, boobs, nipples, genitals, etc ? it would seem to me that any verbiage that accomplished a refinement towards a specific definition would be a good thing.