The compartment is the exact same part as on the Nitro Savage. The large piece is for an optional battery gauge for the receiver battery, the other piece however I couldn't fit an option for. My guess is it's for some sort of charging port. That extra servo location was for throttle control on the Nitro Savage. I've been trying to think of clever uses for it on the Flux and the best I can think of so far is for a mechanical switch for lights or something. (And that seems needlessly complex.)

Edit: I did a little checking. It is a charge adapter port. It's stock on the Savage XL. I think I'm going to see if I can use it as a programming port for the Castle link.