Right, the bit that says 16v on is the plenum and the throttle body is pointing back of it towards the bulkhead. Theres a massive box which goes down the back of the engine which has a rectangle hole on the side where the filter sits and a reducer, which has an oval outlet, screws too it to secure the filer.The resonator then clamps too it (the box you are asking what it is) then the feed from that goes to the inner wing.
All the retards saying its an airbox are wrong. You can remove the resonator and piping and fit a bigger CAF to the airbox but youll have to rig up the end to fit the oval airbox end. Or remove it completely for noise but it'll do nothing for performance.