It is an automatic gear device. If the vehicle smoothly turn on the curve, outside of the wheel have to bemuch more turn than inside of the wheel. Diff Carrier is the parts that make both the amount of wheel different to smoothly turn, when you are driving the uneven
It is usually divided into front axle and rear axle, a small and medium size vehicle has equipped with two, a large vehicle has equipped with three and four. It is called with an axle, the parts which is fixed by bearing box, make wheel or gear rotate on an axis.