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. 

Examples:
post #6574823 (the skirt)

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]].

Examples:
post #6592295
post #3256631