In the Settings menu (gearwheel icon, top right of iS window) the Levels of Eroticism are listed and you can select using tick boxes. You need to deselect Progressive Hotness to do this.
The difference between nudity and full nudity is that nudity doesn't include poses with open legs. This isn't an iron rule, it's a judgement call by Totem editors, so sometimes questionable. Both levels often ignore accessories, such as shoes, hats, jewellery, etc.