It's rude to steal someone's seat.
Tiffany booked a flight and was given an isle seat despite requesting a windowed seat.
Tiffany wants to sit in a windowed seat.
Tiffany sits in her assigned seat and when someone sits in the windowed seat she asks if they would be willing to trade.
Tiffany boards the plane and sits in the windowed seat hoping nobody will confront her about it.
Tiffany's offer to trade is accepted, and she gets to sit in the windowed seat.
The person assigned the windowed seat lets the flight attendant know, and Tiffany is asked to leave the plane.