It is part of your switch matrix. Eliminates wiring.
Study the switch matrix in your manual how it is a big box, laid out in rows and column. Then you might begin to understand. The diode lets the current through one way and not the other.
Which is why if you have a diode on backwards, wrong switches fire. If you don't have a diode, multiples can fire.
Normal switch diode is a 1N4001, I prefer 1N4004 or 1N4007. Those two are used on coils. And can be used on switches and lamps. And no chance of ever mixing up with 1N4001 and putting that on a coil, which would blow up and possibly do damage as it's voltage rating is lower than the solenoid circuit.