Hardware signal demonstration
The external hardware signals are used to control the PDU when you are not using your Bluetooth device. For motorcycles this is very important as you will not want be able to have ready access to your phone when you want to change a circuits characteristics. You will program the PDU from the phone but you will control it day to day with these signals wires which you will connect to buttons, switches, or other circuits like your horn, kickstand sensor, or high beams. When these signal inputs are pulled high or low (12V or 0V) the PDU will get that signal and you will be able to program the response to it. For example, you may set it up so that when the high beams go on your AUX lights turn on, or when the OEM horn is on your air horn spins up.
There are 4 signal inputs that can identify two states each (ground and 12V but not at the same time). So that gives you somewhere between 4 and 8 signals depending on your creativity in designing your setup.