Yup, sub trim is like a fine tuning trim. Really helps you center the trim. I don't have that since I have all cheap radios on my stuff. But it's a pretty cool feature.
Dual rate. The name really doesn't describe what is going on. My take on it is like ATV. Increasing dual rate reduces the total servo throw. It's typically used for steering. A simple demonstration. Turn your radio on. Turn your car on. Turn the steering wheel on the tx. Look at how much the wheels turn. Then turn the dual rate adjustment all the way in the opposite direction. Then turn the steering wheel. You'll notice that the wheels turn a lot less. This allows you to limit total steering response for track conditions. If you reduce steering throw you are less likely to spin out or make horribly extreme overcorrections in your steering. Plus, if you don't need the full range of steering for a particular track, dialing down the dual rate will help you make smoother turns. And smooth is fast...