Differentiating Medium Earth Orbits from High Earth Orbits at the geosynchronous altitude makes intuitive sense. Is there some meaningful difference between orbits above vs. below 2,000 km, or is the LEO/MEO distinction purely arbitrary?
Indeed it was originally based on radiation dose rates. Hank Garrett told me years ago that it was considered the altitude above which satellites in relatively low-inclination orbits had to take serious radiation hazard reduction measures. But that justification is obsolete, and was tenuous to begin with. Since the particles (especially electron and protons) are tightly confined to spiraling along magnetic field lines, both the inner and outer Van Allen belts are at lower altitudes at higher magnetic latitudes. We've learned in the past few years, partly as a result of NASA's Van Allen Probes, that the huge variability of solar activity has stronger effects on the Van Allen belts than previously recognized, sometimes filling the entire region between the bottom of the inner belt and the outer edge of the outer belt, and that often drives the bottom edge of the inner belt to lower altitudes. After the filling event, over a period of 1 to 1-1/2 years the region below the outer belt slowly decays, separating the inner from the outer again, and eventually making a more tenuous and smaller (in radial extent) inner belt. So the altitude of the onset of serious radiation effects varies a lot, to some extent unpredictably (on weeks or months time scales) due to the unpredictability of solar activity on those time scales. Some people might now be looking for a different justification, but there's nothing particularly distinctive about 2000 km altitude.