I suppose, to use technical standards, you have LEO, MEO, and HEO: low, medium, and high-Earth orbit. LEO is defined as below an altitude of a third of the Earth's radius above sea level (so, about 2000km and below), HEO is anything above geosynchronous orbit, and MEO is everything in-between. Decoupling the concepts from Earth (since tidal lock makes orbital period moot relative to planetary rotation as a metric), I guess you could define similar concepts as... low orbit is a third of the planetary radius above the surface of the planet, high orbit is greater than 5x the planet's radius, and medium orbit is everything in-between.
Then you can just cap high orbit at, like... 50x the planet's radius, since the Moon is about 30x out from Earth. Seems like reasonable boundaries to me. Low-orbit would be the usage of ground-support fighters (for which my campaign will no doubt have many), medium-orbit would be having a ship dock at a planet (construction range), and high orbit would be simply being near a planet.
Definitely going to be an interesting campaign, as soon as the 2.0 update rolls around.