The long stem float sensor detects when the water level needs filling as well as a low water cutoff.
This device has two reed switches. A reed switch is a pair of hermetically sealed contacts that open and close with a magnet. The float on this device has a magnet.
There are four wires connecting to two pairs of contacts. When the float is in the up position the red labeled wires will have continuity. When in the down position the yellow labeled wires will have continuity. See the schematic is on page 11 of the HE440 Product Data manual.