And the fare basis, too. The way that BA files DIFs now, and the way that ITA shows them, means that a WT+ DIF's fare basis will be shown with the name of the underlying WT fare rather than what you'd expect for a T class fare. So, having checked (from the itinerary lines) that you're actually booked in to T class, the fare components might be identified as (say) OHX8C1P4 and OKX8C1P4 (to take a random example on LHR-JFK-LHR). These are therefore DIFs - and the YQ is at the lower level.

In contrast, a non-DIF T class fare basis will be something like THX6C1S4.

I have long said that one useful thing to do, if it looks like you will be buying a published fare, is to print to PDF the ITA pricing page and the fare rules pages for all of the fare components in your itinerary. You will always then always have those to refer back to, even though BA doesn't itself tell you very much about the fare rules either at the time of purchase or afterwards. And having the full TFC breakdown means that if (for example) you do a UuA later that you have to phone for, you should be able to work out what the TFC difference is going to be before you call BA, so that you can then check that the agent isn't quoting you something unexpected.