bbp axle change what backing plates are needed

Looks like i need to be educated on this, because after reading through this thread, i'm pretty confused. I am started the process of building up my rear end and will be using bbp axles. A couple of questions:

1) Why would bolt spacing on the axle determine which backing plates to use?
2) If i already have backing plates, how do know if they'll work with bbp axles?