To me All Mountain is doing all the climbing/trail that a full on XC bike would do and on top of that do all of the trails that a full on DH bike would do. Basically go to the trails that the DH bikes would love but cant get too (no shuttling and too far to walk/push). Too difficult for a XC bike to reach (knarly descent before the real climb where a XC bike would excel.). Basically where neither one of those categories can reach...but will excel at is what a AM should be.
I know I may have just described endurance.....but yea