The bottom NC button and top NO button represents the water conductability in the tank(4x 1u-OHM resisistors connected to just form the tank). When the water hits the top(NO button) the motor turns off which represents full at the NO button. When the water drops low(NC button) the motor turns on below the NC button to fill back up intil the NO button. 20M-OHM represents the max resistance of water.
