Replacing ignition with relays, can I combine circuits?

I have a fingerprint sensor that I'm hooking up in lieu of the ignition, and I have three relays that emulate the key.
My question is, why not combine ignition circuits so it's only one relay?

There are only two 12v+ inputs to ignition, as pin 3 & 2 are bridged and are the EU required 'parking lights' circuit, and so must be able to run independent of the rest of the ignition. I have no requirement for that in my country and would never use it so I'm happy combining that circuit.

What I'm wondering about is why 6 & 8 get special treatment. 8 goes to the brake switches, and 6 goes to instruments pin 2. I can imagine brake switches and lights get special treatment because they are fused (whereas the first two ignition circuits are unfused after the 30amp main fuse) but I also don't know what instruments pin 2 is. How do I find that out, as I've gone through the wsm but may have overlooked the info.

It may be I just use two relays instead of the three.