Everyone else has already answered your question.
To expand on dual rate a bit. This is a function of your transmitter (not all transmitters have dual rate capability) not your servos.
Another good use of dual rates is when you can't get the throws as low as you want mechanically. This happened on my Kadet Senior with the aileron servos.It had too much movement on the ailerons but I couldn't move the clevis in any due to binding, dual rates at %50 took care of the problem.