Originally Posted by obduro

Let me correct myself - it's Y on flights operated by TRANS STATES AS AMERICAN EAGLE FOR AMERICAN AIRLINES. Other domestics book into J and you can force a domestic on most routes but it pushes the price up slightly e.g. LHR - NYC via WAS costs an extra £19 return.

The fare code is correct, use ITA Matrix to locate the fare and the rules and fare basis are listed there. Also your info is incorrect, refunds are not free, they cost £150 and you will get the full amount (minus £150) as long as both the outbound and return leg are booked under AQX6C1S4. Extract