I admit that maybe interpreted and this described those tags wrongly. If you look at derpibooru definitions of those tags:
> sideboob - when clothing or positioning reveal only the sides of breasts.
> underboob - when clothing or other covering/hiding reveals the underside of breasts.
and both of them have this note:
> Generally not used on entirely topless characters unless the breasts are partially hidden.
I am fine with using those definitions on Furbooru too.