I think they might be referring to a couple of things:
-some folks don't want a straight, horizontal top hem that contrasts with their pants and ends right at the very widest part of the their hips, because they feel that emphasizes the width of their hips, while if the top is a little shorter, or tunic length, it might not.
Other ways around this are semi-tucking (which forms a downward curve rather than a straight line, or asymmetrical or angled hems.
-also sometimes there is a length of top that looks like it cuts the body in half (50%), rather than visually being at 30% or 40% of the length of the column created by the body. (golden mean proportions and that kind of stuff)
-then there are proportions that work with what you are wearing. My very general habits are to wear tunic length tops with skinny bottoms, semi-tucked tops with mid-rise jeans, tucked or short (high hip) tops with high-waisted pants and skirts.
