When playing the popular 2/1 GF system, the 1NT response needs to be addressed. In the old days, 1NT was 6-10. But, when a 2-level response shows about 13, the responder needs to cope with 11- and 12-point hands. Those are lumped into the 1NT response.
If partner opens 1, and responder holds, say: 5
Q 4 2
K Q 10 8 7
A 6 5 2
, he has to respond 1NT (he doesn't have enough to respond 2, GF).
As to what you call this 1NT response, that's an overrated topic. First of all, the terms "Forcing" and "Semi-Forcing" are a silly pair. The 1NT response is either "Forcing" or "Not Forcing." There is no such thing as "semi." What is meant by "forcing" is that the opener has to bid again--he can't pass the 1NT response. What is meant by "semi-forcing" is really "not forcing." The opener can (and will, with a flat minimum) pass the 1NT response.
My preference is to play 1NT as Semi-forcing--that is the box I check for a 1NT response (on the convention card). I don't think it is a big deal as to what you label the 1NT response. Really, it is just a catch-all bid to show 6-12.
If "semi-forcing," the opening bidder doesn't have to bid again. If forcing, he does. In either case, ACBL requires the partner of the 1NT bidder to make an announcement ("forcing" or "semi-forcing" -- depending on the partnership agreement).
Here is a quick view of the good and bad of playing 1NT as 100% forcing. BAD: If 1NT is truly forcing, then opener must bid again. If opener has a flat minimum, he has to rebid in a 3-card minor (he can't repeat a 5-card major). Though possibly the last making contract, it is impossible to play in 1NT. GOOD: If 1NT is 100% forcing, it allows responder to later show a 3-card limit raise (by jumping to 3-of-the-major) or to bid a new suit on the 2 level to show a weakish 1-suiter.
I prefer "semi-forcing." This means that opener can (and will) pass with a flat minimum. Accordingly, if opener does bid a new suit, he is not flat (he is at least 5-4). Here is a quick view of the good and bad of playing 1NT as semi-forcing.
BAD: If responder has a weak 1-suiter, he will end up unable to show it if 1NT gets passed. Also, responder might play 1NT when he actually holds a 3-card limit raise. In my experience, that's not so bad (often 1NT makes when 3-of-the-major would have failed). GOOD: Because opener's rebid is now "real," bidding is more accurate (opener's new suit is not one of those improvised 3-carders). Also, 1NT is often the correct contract and can now be played.
I've spent 30 years observing that "semi-forcing" is slightly better (and much simpler) than the alternative of 100% completely forcing. If you and your partner want to play it completely Forcing, then mark the card accordingly. Either way, I don't consider this to be too big of a deal.
Notes: The bottom range for any 1NT response is assumed to be 6 points, but, especially at favorable vulnerability, it could be a very weak tactical action. By a passed-hand, 1NT is never forcing. After a minor-suit opening bid, none of this applies.
Additionally, there could be that “in between” “just right” (14 pt 5-3-3-2) hand that wasn’t quite worth an upgrade to open 1NT, but where it just feels wrong to pass a 1NT response. I feel this topic comes under expert bidding judgment. Opener could decide to bid a 3-card suit if he can’t stand to pass 1NT.