Away from home and my own pile of dusty archive material, but in agreement with John_Tullamarine that it's largely a British way of data representation.
'D' was simply a given runway distance ( be it ASDA, TODA etc) corrected for Slope and Wind Component to an equivalent still air / zero slope distance, which could then be applied to standard data for comparison with Pressure Height and Temperature.
Although typically British, much the same is done with American data, but the 'D' tag is not applied.
As for John_T's comments regarding the "typical Brit presentation to have separate AEO/BFL/OEI charts", yes indeed, much more accurate analysis and optimisation was possible.
Sorry, 'X' doesn't come to mind, awaiting return to home's dusty tomes.
Regards,
Old Smokey