After the trump suit has been agreed, a bid of 5NT, bypassing 4NT, enquires about the top three trump honours. There are various modifications of the convention, but the most common are EITHER:
6♣ shows none of the top three honours in the agreed suit 6 of the agreed suit shows one of the top three honours in that suit.
OR:
6♣ shows one of the top three honours in the agreed suit 6 of the agreed suit shows none of the top three honours in that suit.
With two of the top three honours, in both cases responder bids the Grand Slam.