I think there is a coordinate system in the user/developer guide. The units are in ~mm.
Ideally, it should be perfect cartesian system but in practice, I don’t think it is the case or at least how cartesian it is depends on the location. I made the uArm going in a rectangle with set_position and I didn’t moved in the perfect rectangle. Anyway, with a little calculation, it could be calibrated (with some sample coordinates) to be cartesian in the area of interest.