After a positive reply to the 2♣ enquiry (2♥/2♠), a bid of 3♦ indicates a four-card fit in the bid major, values for game but a 4-3-3-3 or 3-4-3-3 hand pattern. Partner chooses either to bid four of the major or 3NT depending on his hand pattern.
Duplication of distribution may mean that 3NT is an easier game contract despite the 4-4 major suit fit. This convention is not in widespread use because many players do not use Stayman at all with a 4-3-3-3 hand type.