I think this is VC10 terminology and, as I was never forced to fly the beast, I don't know a huge amount about it. My guess would be that "D" is a variable that can eventually be turned into a MTOM. You would expect two values of "D", one for all engines, one for engine out and you would take the most limiting to derive your MTOM. I've got some old books in the office, if you don't get a decent reply I'll look through them tomorrow and see if I can answer your question properly and give you some references, The HQ38 Group Performance course notes circa 1976 would be one.