When a character is nude but partially covered by a sheet. The sheet doesn't need to cover any erogenous zones to qualify for this tag (example [questionable] post #151783). If a character is covered to the point where it's impossible to tell if she is wearing underwear under the sheet then use your best discretion. For example, post #873199 and post #508072 are prime candidates for this tag despite being almost entirely covered. The girl on the right in post #850338 on the other hand is not showing enough skin to assume anything. If you're unsure, it's probably best to tag it anyway.
Though thicker, this tag also currently covers blankets. This tag shares some overlap with under covers.