Walking around in their underwear or naked, bathing or sunbathing naked around others, or flashing and streaking. While playing, Sims will learn about nudity and get used to that lifestyle by gaining experience in nudity-related skills.
You can adjust these details by using the Body Selector and by modifying Sim gender flags in CAS Gender Settings.
At any point in time Sims can undress from their clothing, revealing their naked body, which comes with defined details provided by WickedWhims.