Explainer-What is the panel that blew off a Boeing plane in mid-air?

As Alaska Air Flight 1282 reached just over 16,000 feet on Friday, the panel tore off from the side of the jet, leaving a neat, refrigerator-sized, rectangular hole in the aircraft. The 737 MAX 9, currently Boeing's largest single-aisle plane, can seat up to 220 people. Of the 200-plus 737 MAX 9 planes Boeing has delivered, 171 have this panel in place of a door.