Welcome to the forums. Generally it works as follows: After a 1NT opener, people play 2♠ and 2NT as transfers to ♣ and ♦, respectively. Since 3NT is usually a more desirable contract than 5m, it's useful for opener to have the ability to show whether or not he has good support for the minor, because if he does, then responder's minor becomes a good source of tricks in 3NT. Thus, people "bid the gap" to either say they have fitting cards for the minor or that they don't have fitting cards, depending on your agreements with your partner. After that, 3 of the minor is generally used as a drop-dead bid, 3M is shortness in that M, and 3NT is to play.