Used when a female character is in a situation where you would expect her to be wearing a bra (or equivalent underwear), but she isn't. Note the following corollaries:
no_bra should normally not be used together with the nude tag. Even if the character is nude in a seemingly inappropriate situation, not wearing a bra is always appropriate in the context of being nude. Use no_bra when the character is wearing clothes, but still no bra.
no_bra can be used together with bra; this can happen if the bra is present in the picture, but not worn.