Oo, didn't read that part of your reply Peloton, but yeah, I agree 100%...it's a place to mount the license plate bracket. The bracket is a rail with two sliding clips. The clips are slid out along the rail until they butt up against sides of the hole and grip from the inside of the bumper, while the rail presses in from the front. The clips are then secured by tightening their screws so that they don't move. The license plate is, in turn, fastened to the rail on the outside.
It's a simple and effective mechanism and allows you to attach plates of any size.