We have three sub-categories at the category 'Nude': Please check out FAQ for detailed information.
1. Nude Skins
Short: Skin with sexual Characteristics.
2. Clothes for Nude Skins
Short: Let's your Sim show his Skin. Is needed when you use a nude Skin.
3. Nude Clothes
Short: Needed when the Sim does not have a Nude Skin but you want him to be naked.
Resume: If you or rather your Sim does not have a Nude Skin for a certain skin colour or no nude skin at all then you need the Nude Clothes to make your Sim naked.
If your Sim does have a Nude Skin then you'll need the Clothes for Nude Skins.