Originally Posted by
El Bunto
Which wouldn't mean anything to ATC since the flightplan says KSEA
Well it does mean something to most of us in ATC as we do know the IATA codes, well most of them anyway, just from familiarity really as there's no requirement for us to know them. Also the airfield often has a VOR with the same 3 letter code, e.g. DUB (although that VOR isn't actually on the airfield).
The IATA and ICAO codes tend to be the same for North America anyway, CYUL, CYYZ, CYVR = YUL, YYZ, YVR. KJFK, KMIA, KLAX = JFK, MIA, LAX.