The UGS stock rail provides indexing and length adjustment that the rail on a buffer tube would normally provide. By swapping between the different rails on your UGS you are able to reconfigure the stock for the use of Milspec CO2 inserts and Milspec stocks, or 2.000" diameter HPA tanks and 2.000" stocks such as the G&P R3 and similar stock used in paintball applications.

All UGS stocks for M4 models use the "Original" rails.
All UGS stocks for HK417 models use the "R01" rails.