Originally Posted by
Antarius
It is the IATA code for a 777-300ER. There was a 777-300 also produced (a stretch of the 777-200) which is a 773 and that's why it has a different moniker.
Similarly a 777-200 is a 772, 777-200ER is a 77E and a 777-200LR is a 77L.
Though I must admit out of all the codes listed - 773, 77W, 772, 77L - I’ve never seen 77E until now. In most instances, 772 or 777 is used regardless of the ER or non ER version.