Globaliser

Dual Inventory Fare.

I think that the way they work is like this: Normally, you would expect a T class (WT+) fare to have a fare basis code starting with the letter T, for example TLX8S4C1; normally, the first letter of the fare basis indicates the booking class into which the fare must book (at least on the main part of the journey).

However, KARFA quoted a fare basis QLX4S4T1/DIF4. If you were to see a fare basis of QLX4S4T1 on its own, you'd expect it to be a cheap economy fare booking into Q class (the second lowest booking class) - and if you look at a fare listing for your route (eg on ExpertFlyer) you should be able to see that fare listed.

However, QLX4S4T1/DIF4 is a fare that books into T class and therefore gets you seated in WT+. But it should only be valid if both Q class and T class seats are available on the flight - hence "Dual Inventory". If Q class is sold out on your flight, then it does not help if the flight is T9. To book in T class, you'd then have to pay a different (and higher) fare. For example, if the flight is T9 N9 Q0, you could book in T class using NLX4S4T1/DIF4 but not QLX4S4T1/DIF4.