Dual rate is a way to make the steering more agressive or more soft.
At neutral, 100% wheel turn on the TX corresponds to 100% steering throw on the car.
At 80%, 100% wheel turn on the TX corresponds to 80% steering throw on the car.
At 120%, 100% wheel turn on the TX corresponds to 120% steering throw on the car - not good, unless you limit the servo travel, either mechanical or on the TX, if your TX supports it.
More practical, on the track, set the dual rate so the car can turn sharp enough for the track AND your driving style, but also so the car dont feel twitchy.
Schumacher Mi5 - Sanwa MT-4 / RX-462 / KO Propo PDS-2123 - Nosram 5600 X-treme Saddle Pack - LRP SPX Zero/X12 17,5 - Nosram Stealth Touch Evolution - MAAS SPS 9250