Basically, there are a bunch of parameters for an aircraft( wingspan, planform area, gross weight, propulsion type, etc) and these programs take them and develops the velocity envelope in which the aircraft can fly as well as the max endurance, range, and flight ceilings. There is a min and max velocity at a given altitude due to thrust required and thrust available which is the upside down u shaped curve. Then there is the stall speed for the aircraft at each altitude, which is the absolute minimum velocity that the aircraft can fly before it’ll drop out of the sky. These charts show the speeds possible for steady level flight of the aircraft at altitude.