It’s called ‘rain sensor’ port but it’s really a general purchase sensor port. As you may know, it can be software configured to rain sensor, flow sensor, or a program start switch. I was suggesting it because it’s a terminal that you can plug in a sensor to, and then just change firmware to customize its functionality. You can certainly use any of the spare GPIO pins to read sensors too, but that requires opening the case and solder a wire or two to the circuit board.