It’s so bad that my fiancée has some bras that say she’s a B cup and others that says she’s a D cup. In order to go bra shopping, you have to actually try them on to find out if they fit.
If I had to try on underwear to see if they fit, I might not bother with underwear at all!
Boot cut is tighter around the hips and thighs, has a slightly flared leg (think bell bottoms but way smaller flare). Relaxed is straight leg, but not so tight as to be skinny jeans. I generally wear boot cut, but my boots are only ankle high so it’s not like I need the extra leg room. I just like the way they look on me.