This reserve indicator works together with every potentiometer based fuel sensor. You can store 2 different values of fuel in the module. The included 5 mm LED flashes if the fuel amount is below of the first value. The LED is continous on if the fuel value is below of the second value. 

The adjustment of the reserve level is easy: Just connect the module, fill the first required amount of gasoline in the tank and hold the green cable for a moment to ground / Minus. Now the module have store the first value. Then fill the next gasoline level in the tank and repeat the procedure and the module have stored the second fuel level. That´s it :-)