RE: Subtrim versus Trim
Ok, Let me take a shot at this answer.
To center a servo you need a 1500us pulse width signal from the transmitter.
With this signal sent, the position of the control surface may not be centered with the airfoil.
Also, if you have multiple servos on the same surface but different channels, they could be fighting.
There are two ways to adjust this, the tried and true way is to adjust the clevis to the point where the control surface is centered and any binding is eliminated. Computer radios allow you to adjust the sub-trims to get this same result by making a position other than 1500us as the center of the servo. Now, because the servo can only handly 900us to 2100us and adjustment from 1500us will make the end of travel shorter for the side the center position moved to.
I hope this clears it all up.