The other trim is the mechanical trim of the heli. With a servo/linkage/slider/variable pitch t/r set up, the relationship between servo postion and t/r pitch is adjusted via linkage length. This is the mechanical trim. The TX trim/subtrim can adjust for only so much mechanical "un" trim. Generally, minor TX trim is OK but if the trim is all the way to one side and you're still having to lean on the stick, you really need to think about getting the mechanical trim set up. You probably need to get the mechanical trim set correctly first before you start touching the TX trim/subtrim.

If the heli trim is electronic (electronic motor or coaxial rotor) then the controller on the heli needs to be adjust (electronically equivalent to mechanical trim) to achieve close to correct trim before touching the TX trim/subtrim.

Good Luck,
d.tipton