Yesterday on a flight with my wife, I was in 4F and she in 5F. The AAngel at the AC told me that 4E had not checked in as of 45 min before the flight, and to ask the gate to release the seat.

I asked the gate AAgent if they could release 4E 30 min before departure (and swap whoever it was to 5F), and she looked it up and said "That's an EXP in 4E, so I can't release the seat until 10 minutes before departure."

EXP, whoever it was, no-showed, and my wife ended up in 4E, and a non-rev uniformed purser ended up in 5F.