Sub trims are different from the trims on the face of the radio. When trying to set your servo arms up at 90 degrees as most instructions say can be challenging due the servo arms teeth not mating properly on the servo at 90 degrees. This is when you use the subtrim not the standard trim. The subtrims allows you to get that servo arm at 90 degrees without going crazy.

Caution: If you need more than 20% subtrim in either direction it's better to take the servo arm off and move it one click on the servo and then reset the subtrim to 0 and try again to get the servo arm to 90% with the subtrim.