It was the model designation same as L/LX usually high spec 4 electric windows, sun roof and a couple of lolly sticks of wood looking plastic in the door trims
If it's on a separate plate, and not part of the maker's model no., it means Corps Diplomatique (Diplomatic Corps) and indicates that the driver can't be fined for parking on yellow lines - in fact can get away with almost anything.
I was under the impression that if the model has a D in the name it means it has a diesel engine. I'm no expert on cars though so I could very well be wrong.