This page allows you to set which trim system will be used for this class of aircraft. Usually you will want to select FS Force. However, if you have a flight yoke as well as a force feedback joystick, you may wish to use the flight yoke exclusively for certain aircraft.
Just trying to wrap my head around this. I have assigned the Saitek Trim wheel and Iris Dynamics Yoke in Prepar3D and selected FS-Force to handle the trim system. What is the advantage of providing the 2 systems instead of providing FS-force as the primary trim handling system? If trim is not intercepted by FS-Force are you saying that the centering force will not be felt on the yoke if FSX is selected?