Paraphrased from "The Encyclopedia of Bridge" 1984 ed.

4NT DI promises

1. 2 aces by an unlimited hand, or
2. 1 ace by a limited hand

It asks partner to show an undisclosed feature (like an Ace, King, singleton, etc) by bidding that suit.

Partner's bid does not show extras, unless it goes past 5 of the trump suit.

Responder may also jump to 6 of the trump suit to deny grand slam interest or rebid 5NT to show a complete maximum and an interest in a grand slam.

Over a 5 level response, a 5NT rebid asks for another feature (for a grand slam) and promises one more ace than originally shown.

4NT is Blackwood if

1. it is a jump bid, or
2. it is bid immediately after a sudden leap to game