ATV is adjustable travel volume, this does the following:
In steering it limits the maximum amount you can turn the wheels (say 15 degrees rather than 20etc). This is useful if you are on a very loose surface where too much steering can cause the car to oversteer.
in electric throttle make sure its set at maximum otherwise you will lose full speed.
in gas throttle it sets the point at which the throttle valve is opened fully and doesn't give you a dead band where nothing happens.
Dual Rate alters the steering sensitivity. instead of having a linear relationship between the controller and the actual car, the realtionship becomes exponentional. best illustrated by an example.
with linear (dual rate =0%)
5 degrees movement on the steerwheel causes 5 degrees on the servo
with dual rate =100%
5 degrees on the steerwheel causes 12 degrees of movement on the servo.
the actual maximum amount the steering moves is the same in both cases, but the dual rate causes more sensitivy nearer to neutral.
i hope this helps, as it may be a bit confusing.