Originally Posted by WhySoTough
Also I remember reading somewhere that packs off isn't allowed in very high elevation to risk forgetting to turn them on or something which will cause aircraft to be unpressurized. So yes air conditioning and pressurization.
The higher the airport elevation, the greater chance the aircraft can't takeoff with engine bleeds on, since the performance limited takeoff mass (PLTOM) is lower due to higher density altitude and one way to increase PLTOM is to perform a bleeds off takeoff.