How does the AUTO Player # work?

I usually always just manually set the player number for CDJ's when setting up players for festivals. but i'm actually curious how the Auto selection works. and how accurate it is. Is there a certain sequence of things I need to do to assure it will get the correct player number relevant to the channel it's plugged into?
What if you are using both RCA & Digital outputs, perhaps on different channels (not usually something that happens, but good to know what the outcome would be)

Using CDJ2000Nexus or CDJ2000NXS2 into a DJM900nexus or DJM900NXS2 mixer