Full throttle height - the height an aircraft can reach whilst maintaining the maximum permitted MAP for the engine. Above FTH the MAP will start to drop because the supercharger can no longer compensate for decreasing ambient air pressure to maintain that maximum permitted MAP. Example below is for a P-40-N, FTH is marked with the asterisk *
Other aircraft/engine operational settings will change the FTH but MAP will always start to drop above FTH even though the throttle is wide open. The aircraft will climb above FTH of course but from then on it is a losing battle between the supercharger and ever-decreasing atmospheric pressure. Service ceiling is, I believe, the height reached when rate of climb is down to 100 fpm and absolute ceiling the height reached when rate of climb becomes zero - again, see below.
