Gauges and Senders communicate through the resistance from the sender. There are 2 ways to measure resistance from the senders, European Standard & American Standard.
European Resistance Measurement: Has an output of 10 Ohms when empty. As the fuel level increases, so do the Ohms.
American Resistance Measurement: Has an output of 240 Ohms. The resistance will decrease as the fuel level increases resulting in an improved accuracy.