From my old Ground school texts: The breaks for elevation are ground to 3k feet and 3k to 10k (above which you need supplemental air) , 10k to 30k Pressurized cabins or significant breathing and clothing requirement) and 30k and above (pressurization is pretty much a must). This complicates the aircraft and more of the weight and interior are required for keeping the crew alive. Airliners us above 30k since it is generally smoother and their engines are more efficient at cruize. Also it is above most flack or SAMs for combat, but that ,as we saw recently is not always the case.