Responding to 1 Level Suit Opening Bids:
A 1NT response to a 1C, 1D, 1H, or 1S opening bid is GAME FORCING, with no reference to any particular suit or denomination. Bidding after this continues in the normal way, but a rebid in a major shows at least a six card suit, 2C or 2D should be natural but can be a good three card suit, though with a balanced 5332 hand the rebid should be 2NT
In such a game forcing situation, any suit agreement or a new suit by the 1NT responder, can be treated as a KEY CARD ASK, which will be covered later