It is not used after a minor suit opening.
In standard bidding, it is a catch-all bid for all hands with 6-9 HCPs, without support for partner's opening and not able to bid 1♠. With 10+ HCPs and no support for partner's opening, a responder bids their 4-card suit at the 2-level, or 2NT if the hand is 10-12 HCPs balanced and there are stoppers in all unbid suits (unless Jacoby 2NT is used).
However, in the 2/1 game forcing system, a 2/1 response promises game-going strength (i.e. 12+ HCPs) by an unpassed partner, therefore, the original 2-level responding cannot be used by hands with 10-11 HCPs. Therefore, the 1NT response is expanded to all hands with 6-11 HCPs, without support for partner's opening and not able to bid 1♠. It is now forcing for one round.
As a passed hand cannot force to game, some plays a variant called semi-forcing 1NT if the responder has passed which allows opener to pass with a balanced hand and a bare minimum.
Opener rebids Edit
- With 6 cards in the original major, bid it again at the 2 level (jump to 3 level if holding a strong (18+) hand). 
- With 4 ♥s and opened 1♠, bid 2♥ if weak to intermediate or 3♥ if strong.
- After a 1♥ opening, a 2♠ responses promises 5 ♠s (since responder would have been bid 1♠ if holding 4 ♠s).
- Otherwise, bid the longest minor. If they are of equal length, bid 2♣ if 3-3 or below in the minors, 2♦ if 4-4. Therefore, a 2♦ response promises only 3 ♦s (all 5-3-3-2 shapes with doubleton in ♣), a 2♣ response promises only 2 ♣s (4=5=2=2)!
- With a balanced hand and stoppers in all suits, bid 2NT if invitational and 3NT if strong. If the semi-forcing variant is used, opener may pass with a minimum.