After a 1NT opening, a 2 response asks opener whether he is minimum or maximum for his bid. Opener responds 2NT with a minimum or at the three-level in his lowest four-card suit with a maximum.
Following an initial 1NT move, a 2 reply from the responder asks if the opener is at the lower end or upper limit of his bidding range. The opener then responds with a 2NT that signifies either the lowest possible bid or the highest feasible bid in his weakest suit.