Sub trim will allow your Tx in flight trim to stay centered yet physically position that center point for a given control surface. Actually if you are setting up to install push rods and your two asymmetrical servos (aileron servos), use your Tx instead of a servo tester. You know when you center your two servos and put them side by side, no matter how you try to fit the servo horns to them one is always off. It is because the splines that lock the position is usually an odd number. You can counter this by using sub-trim, in your case having the DX6 it does it on the controller