I don't recall seeing one specifically about DIFs.

The concept is really quite straightforward. Take a number of hypothetical flights on which the WT+ and WT availability looks like this (the reason for the bolding will be obvious later):-

1. W9 E9 T9 Y9 B9 H9 K9 M9 L9 V9 S9 N9 Q9 O9
2. W9 E9 T9 Y9 B9 H9 K9 M9 L9 V9 S8 N6 Q1 O0
3. W9 E9 T9 Y9 B9 H9 K9 M9 L6 V3 S0 N0 Q0 O0
4. W9 E9 T9 Y9 B9 H9 K9 M6 L3 V0 S0 N0 Q0 O0
5. W9 E9 T0 Y9 B9 H9 K9 M6 L3 V0 S0 N0 Q0 O0

The cheapest WT+ fares book into T class.

Consequently, a standard WT+ fare (ie one that is not a DIF) can be booked onto any of those flights except flight 5, for the straightforward reason that there is no T class on that flight.

However, BA may also file a number of T class DIFs. Each of them will require availability in T class, in the normal way. But in addition, there must be inventory in a specified WT class. So (hypothetically) there might be DIFs for this route that require availability in:-

1. [*]
2. [*]
3. [*]

As you go down that list, the fare will go up.

If you try to book on flight 1, the cheapest DIF (fare A) will be valid, because there is both T class availability and O class availability.

If you book on flight 2, fare A will not be valid because there is no O class on that flight. However, fare B will be valid because there is N class.

Similarly, on flight 3, neither fare A nor fare B will be valid, but you may be offered fare C because there is V class.

And on flight 4, the only DIF that is valid is fare D because L class is available but nothing lower.

It goes without saying that none of the DIFs is valid on flight 5 - not even fare D - because although there is L class in WT, there is no T class in WT+. Fare D, just like the standard non-DIF fare, requires there to be T class availability as well as availability in the specified WT+ class.

One of the frustrations with BA's current implementation of DIFs is that if you look at the EF fare ladder, it is impossible to tell which underlying WT booking class is required for that fare. You can sometimes work it out by trial and error by looking at different flights with different WT availability and matching the fares on the fare ladder to the fare quotes for each flight. But I have since decided that life is too short and gone for rather more empirical ways of checking that I'm getting the fare that I expect to get.