You are a bit wrong i am afraid.
EPA is END POINT ADJUSTMENT. This means the amount of movement in each direction the servo will move, or could be the throttle and brake on electric.
Its very important to set the steering so as its not got too much movement that it hits into the hubs or wishbone. as you found you can decrease to have less steering but its only really for beginners and for onroad you look for that!
Dual Rate is a switch that you can set to have 2 sets of adjustments.
one could be with lots of expo or less EPA. its really more common for flyers to use Dual rates and not so much for cars.
I would keep it simple and set EPA to max on the throttle unless its a beginner then it could be good too use.
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