This tag should be used when a character is partially undressed relative to what they are wearing. This means that if the character is wearing a shirt, and part of it is taken off, revealing a bra, then this tag would apply. If a character has a partially removed bra revealing or practically revealing the breasts, then this tag would also apply.
This tag could also be used for the implied removal of clothing, such as when the bottoms or top that a character would normally wear are not depicted on the character. Which could also be tagged as topless/bottomless.