ATV is where you limit (or increase in some cases) the servo's maximum travel.
This helps stop the servo from straining at full steering lock, for example, or lets you reach full throttle in a gas car, if the servo as standard isn't fully opening the carb.
It's the "first level" of setting up the servos in a car.
If you want to tune the amount of steering throw you have, for example, DON'T use the ATV (also called End Points/EPA) control. You should use the steering Dual Rate (D/R), for example setting it at 80 or 90 percent to get less steering on the track.
Dual Rate is the "second level" of servo adjustment.
ATV should be set with the Dual Rate at maximum.