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 means a slightly flared leg (think a very very subtle bell bottom) so that they fit over cowboy or work boots easier.
Relaxed fit means it’s made a little bigger in the seat and thighs compared to the same size in a regular fit. The lines/silhouette aren’t as clean/nice, but it’s comfier; which is why it’s called relaxed fit.