Thanks for searching! ^^

Not sure what the exact difference is but I have always thought of J fares to be the fully flexible BF fares (similar to A fares in the domestic market or C fares for international destinations not marketed as BF).

Likewise, D fares seem to be almost full J fares in that they're still flexible, not refundable and slightly less expensive. I have seen D fares in both international markets and the domestic markets.

That said, this is just a shot in the dark to be sure to take it with a grain of salt.

Bonus wise you'll still get the extra 50%eqms and the 50%rdms