I've always explained the boundary as the point at which, in order to use wings to provide enough lift to keep you in the sky, you have to be travelling at orbi

I've always explained the boundary as the point at which, in order to use wings to provide enough lift to keep you in the sky, you have to be travelling at orbital speeds. As the air gets thinner you have to travel faster in order to displace enough air to keep you aloft, and there comes a point where you're in orbit anyway, so the lift isn't needed. Different planes have different lifting surfaces, so I have always wondered if the line was set more or less arbitrarily using some 'standard' plane, or if all planes approach the same limit together. Looks like there are some interesting things to read here!