D/R is just a switch that you can select in flight BUT you program the switch for the amount of rate on the ground. Assuming High rate is 100%, you can set low rate for, let's say, 50%. If at a later time you were to decide you want low rate to be 70% you would want to reprogram the switch while on the ground, not in the air. While it could be done in the air, chances are you would crash due to the distraction.
The same is true for any other program changes. I have heard of people trying to do it in the air, this is the only reason I mention it. Hopefully you would not have tried it.
You will understand better once you have the radio in your hands and the manual. Read the manual I linked to you before