Originally Posted by
MADPhil
I should have looked up the term but this is also the explanation for why my discounted transatlantic F/A is combined with a domestic Y/K. It seemed quite illogical when I was given that explanation by an agent as the web site explanation was that I was booked into the nearest equivalent class in the absence of F.
I rather suspect something different must have happened in your case.
In BA's implementation, I haven't seen different booking classes used simply because a DIF is valid and used to fare the ticket. If you book a standard (non-DIF) T class WT+ ticket on an itinerary like that, the short-hauls book into K and the long-hauls into T. If you buy a DIF, the same booking classes are used.
I've just tested this on an EDI-LHR-JFK-LHR-EDI using an A class DIF. That uses J class on the short-hauls and A class on the long-hauls, in the same way as if you book a non-DIF A class fare.
So if your short-hauls have booked into K class, I tend to think something else must have happened. I can't get ITA to combine any A class fare on the long-hauls with a K class fare on the short-hauls.