You may want to be more descriptive, I get what your saying I think. You're wondering if the rating for FPSC has partial nudity on it because of a model pack or because of user-content.
First of all, games are never rated off of user-created content. I don't think there were any model packs when the ESRB (or any other rating company for that matter) rated FPSC. I think it's because of one model in the game, that is just a simple corpse with no clothes on. However, nothing is shown at all.
If it is because of a different model, feel free to correct me folks.
Have a good day christianx22, and welcome to the forums!
(edit: I didn't realize that you guys had posted already, oh well.)