What's the thing in the middle of the back cover?

I'm hoping it's an inductive charging thingy, which would hint at back covers having more intelligence and the potential of a HWKB "other half" for those that want it, but I can't see any metal contacts on either half. How would that work? Am I missing something? Is it just an NFC tag to say what colour the plastic is? Let's hope all is revealed at the event later.