Magnetic reed float switches open or close a circuit with only a small change in liquid level - usually a 1/4" to 1/2" movement in level will change the status from open to closed or vice-versa. Where it is desirable to turn a device on at one level and off at a significantly higher or lower level, two float switches can be used.
Suppose that you want a pump to fill a chamber to the point where it is 75% full. You do not want the pump to begin refilling the chamber until the liquid level has dropped to the point where it is only 50% full. The difference in these two levels is 6 inches. Since a float switch activates in about 1/4"-1/2", using a single float switch will not achieve your objective.
The solution is achieved by using two normally-closed float switches. The illustration below shows a simple way to utilize a standard, inexpensive eight prong DPDT relay to prevent the bottom switch from opening the pump circuit and prevent the top switch from closing the circuit. Using this arrangement, the float switches can be placed at any desired "turn pump on" and "turn pump off" levels.
The above illustration is for a "fill-up" application, but the same logic can be utilized in a "drain-out" application. In such an application a pump would be used to pump liquid out of a chamber when it was 75% full and continue pumping liquid out until the level was reduced to 50% full.
You can achieve this by inverting both switches and reversing their position - i.e. the switch shown in the upper position in the illustration would instead be placed low in the chamber where the pump should turn on; likewise, the switch shown in the lower position in the illustration would be placed high in the chamber where the pump should turn off. Remember to invert each switch so that it acts as a normally-open rather than a normally-closed switch.