I dug around the net a bit, and read that indirect drive is when the output shaft is supported by bushings or bearings at both ends, rather than the potentiometer at the bottom. The "pot" is then gear driven off off of the output shaft. They say the benefits are better support for the output shaft, as well as better protection from vibrations for the pot.

I've never seen a servo like that before, didn't know they existed until now. I do have some 3003s that are not indirect drive though.