Subtrim is sort of a "hidden" trim setting. This allows you to center your servos without changing the main trim settings. I use subtrim to level all of my control surfaces visually on the bench, and then use the regular trim to level out the plane in flight. When setting up a plane, I usually center the subtrim & trim, then adjust the linkages to get the control surfaces where I want them, then use subtrim if I'm a little off at the end.