Redshift

1NT:2♣ is 3-card Redshift, requesting the 1NT bidder to reply with his cheapest 3+ card suit — i.e. 2♦ or 2♥. Responder has one of:

6+ club suit and a weak hand (i.e. 0-9 HCP), planning to bid 3♣ on the next round whatever opener replies. (K1#4)
Weak hand (i.e. 0-9 HCP) and 5-card heart suit, when he passes 1NT bidder's 2♥ or bids 2♥ after 1NT bidder's 2♦.
Weak hand (i.e. 0-9 HCP) and 5-card spade suit, when he rebids 2♠.
Balanced or semi-balanced hand containing 10-11 points, when he will rebid 2NT. (R3#3)
Good hand (i.e. 10-13 HCP) and 5+ card suit in ♦, ♥ or ♠. He will rebid the suit at the 3-level whatever Opener responds. (K1#3)
Very good hand (i.e. 14-16 HCP) and a 5+ card suit. He will rebid the suit at the 4-level whatever Opener responds.
Or exceptionally, Weak hand (i.e. 0-9 HCP), at least four cards in each red suit, and a shortage in spades (i.e. either a singleton or void), when he will pass whatever the 1NT bidder replies, preferring to play in, at worst, a 4-3 trump fit rather than NT with a spade shortage.