The FMM has four ports where you can plug in various sensors and accessories (all sold separately). The best part is that they do not need to all be of the same type. This means if you want to have a flow sensor in ports 1 and 2 and then a leak detection probe in ports 3 and 4 – NO PROBLEM! Want a flow sensor for your return pump, an optical sensor for emergency sump level detection, and two leak detection probes, NO PROBLEM! Just plug them in and the FMM will automatically sense which type of sensor you have in each port.