Not saying we would start the sequence with the given hand, but: After the 3-bid by opener, we use 3NT as denying an Ace, but showing one or more kings. So, if we did have a hand with that AJX of Diamonds and had started the sequence, then-- 4D=asking bid over 3NT...second step (4S) would show stiff or KQ(X+) of diamonds. Re-ask would clarify which. A very old Hardy/Walsh thing.