What is this open panel at the rear of this airplane? I saw this photo at another interesting question:



There's an open square panel. What is it?

P.S. Any tag suggestions are welcome. All I could think of was "tail".

EDIT: are there any other tags for this kind of thing? The APU tag seems to be a crystal ball because I had no idea what it was at the time.
It is the Auxiliary Power Unit (APU) inlet door.



Image from Boeing Aero Magazine