It is how strong Bob will be when he pits. If you set it to off then he will just go until his tires are worn out or his fuel is low. This is best with high level drivers. However as for the 10% and other values I believe that is how much strength Bob will have left when he will come in. So at 90%, he will pit when he has lost 10% strength. If you set it at 10%, then he pits when he has lost 90%. Hope this helps