What I like about 100 km is that it’s place where two different fields independently agree that space begins. The Karman line is an aeronautic definition, but space physicists also define the space boundary at about 100 km based on composition. It’s about where the ionosphere begins and the mesosphere ends. As a physicist, I had never heard of the Karman line until somewhat recently, so it was interesting to learn that the what many people use to define the boundary for space is the same as what I knew it to be.