Is this actually a well-defined altitude, or does it depend on the properties of the vehicle? (Certainly, the minimum speed to maintain level flight normally do

Is this actually a well-defined altitude, or does it depend on the properties of the vehicle? (Certainly, the minimum speed to maintain level flight normally does.) The Karman line is near but distinct from the boundary between the thermosphere and the exosphere, which is well-defined: it's where atoms become so thin that their mean free path becomes so long they stop bouncing off each other and just follow parabolic trajectories under gravity.