The other day I made a question about the External Power Hatch. Now I would want to know what's that hole below the door.
Outflow valve, controls cabin pressurisation. To pressurise the cabin, close this valve. Pump air into the cabin, stop the air flowing out, cabin pressurises. The valve doesn't completely close, but is about 98% closed. You still need some airflow going through the cabin. The cabin is pressurised to give an approximate equivalent altitude of 8,000 ft, whilst the aircraft is flying at 30,000 ft+.