This type of images has a much closer relationship with sexual satisfaction even though they may not be revealing much of anything, it does need a special category because of the association. I know many models who will do a full nudes but not in any way cover their genetalia with a hand as it might be percieved they satisfy themselves. The images will show less than full nudes but still be more ‘risque’ in many peoples opinion.
I very rarely work in this category.