If I understand correctly, the Magpul URB can't be to adjust along the buffer tube, just with the fine adjustments?
Negative, you have those backwards. The PRS is the stock with the fine adjustments. The UBR has a cheekweld that stays in place, but the stock will adjust back and forth on the buffer tube. The UBR also allows you to preset the stopping point. That way, when you set the "stop" at the 4th indexing point you can then contract the stock to make the OAL of the gun as small as possible. You can then extend it quickly and it will automatically stop at the 4th indexing point instead of passing it and going all the way to the rear like other retractable stocks.