Hello, I'm not an aerospace engineering student but I'm interested in aviation and I had a question. My friend and I got into a debate last night about VD. I believe he said that meant vertical descent. Anyway, he said that if an aircraft has a VD limit = 420 KCAS to 17,000ft/.91M that means it can fly up to that speed from 0ft-17,000ft. Is he correct? Also, I know what 420 knots is about 480 mph but what does .91M mean? Thanks.