GEO - Geostationary equatorial orbit (further from the earth, more efficient in terms of energy required to maintain a fixed position[1])
SOL - Speed of Light (a limiting factor in fiber optic communications - and over the air communications as well)
[1] From reading a bit, maintaining a fixed position relative to the ground at lower altitudes involves a constant velocity adjustment in the form of fuel expenditure - the equilibrium for each orbital period corresponds to a single altitude, so different orbital periods (when minimizing fuel usage) imply different distances.
Re [1], you generally can't do fixed positions anywhere else but GEO. Trying to stay stationary at a lower altitude would require using amounts of fuel that are way beyond what we can put in space.
Note that GEO satellites do carry fuel with them for "stationkeeping" - between errors in navigation, variation of Earth's gravity (due to it's irregular shape and composition) and influence of solar radiation and other celestial bodies, those satellites do have to occasionally and very slightly adjust their orbit to stay stationary.
LEO - Low earth orbit (close to the earth)
GEO - Geostationary equatorial orbit (further from the earth, more efficient in terms of energy required to maintain a fixed position[1])
SOL - Speed of Light (a limiting factor in fiber optic communications - and over the air communications as well)
[1] From reading a bit, maintaining a fixed position relative to the ground at lower altitudes involves a constant velocity adjustment in the form of fuel expenditure - the equilibrium for each orbital period corresponds to a single altitude, so different orbital periods (when minimizing fuel usage) imply different distances.