Those functions are to set up the radio for your preferences.

Subtrim is only used if and only if you can not center your steering or throttle by adjusting the linkges, it will allow you to adjust the centering electronically but that will offset the final travel.

Expo or exponential will adjust the feel on either side of nuetral either soften the effect or make it more aggressive.

Dual rate can be used to reduce the amount of steering throw you have.