The FMM (Fluid Monitoring Module) is a multi-purpose module that, with the help of various accessories, enables your Apex to do:

Flow Monitoring
Leak Detection
Water Level Detection
Power 24VDC accessories
Future sensors

Return pump

Measuring the flow of your aquarium’s main return pump is an incredibly useful piece of information. The key benefit would be to have the Apex notify you the instant the flow stops for any reason — usually the result of a failed or faulty pump. Furthermore, an indication of a reduced flow rate could be used to determine that the pump or the lines are dirty or are becoming clogged with algae, debris, animals, or the growth of organisms inside your plumbing.

Auxiliary pumps on reactors and the like

This is a very handy application for the FMM. Place a flow sensor on a device like a carbon/gfo/biopellet reactor and it serves many purposes. First off, you will know if your pump is not not flowing correctly or failed altogether. Next, you can easily fine-tune the flow into the reactor by watching the dashboard tile on your Apex. Finally, you can know when the reactor is becoming clogged with detritus or possibly channeling has occurred by an alert on your Apex or by monitoring your dashboard.

Put a flow sensor inline to your device, plug it in to the FMM, and see exactly the flow rate of the water (any aquarium fluid really) going through that sensor. The industrial-grade flow sensors use either a turbine or paddle-wheel mechanism (depending on the model) to measure the flow.

It is important to note that the flow rate that will be reported back to you from the flow sensors will likely be far lower than you expect or what is printed on the pump label or documentation. This is due to head pressure loss from physical height differences, long runs, restrictions such as ball valves, fittings like elbows, tees, and reducers, and then finally the loc-line many use as the entry point into their tanks. As an example, a pump rated at 3200gph will likely be showing less than half that amount in nearly all applications.

With the FMK – Fluid Monitoring Kit you get one 1? Flow Sensor as well as two 1/2? Flow Sensors

The FMK comes with three flow sensors but the FMM has a total of four ports. This means you have one port free to use for another purpose.

Useful additions - 

1inch threaded adaptor (female)
https://charterhouse-aquatics.com/shop/aquatics/fittings/female-adaptor-sockets/adaptor-socket-female-32mm-x-1

1.2 inch threaded adaptor (female) 
https://charterhouse-aquatics.com/shop/aquatics/fittings/female-adaptor-sockets/adaptor-socket-female-20mm-x-1-2