Instead of reserving a 3NT opening for those very rare 25-27 point balanced hands (which are easily described using the strong 2C openings - 2C pass 2D pass 3NT), 3NT is used in the following instance:
1. A solid 7 card or longer minor suit (the suit must be AKQxxxx or better)
2. No significant values outside the long suit.
Partner passes with stoppers in the remaning suits. Otherwise, responder bids 4C to find the minor and stop short of game, or
bids 5C with the intention of playing game in the long suit.
For a gambling 3NT overcall, the bidder must not only have the long, solid suit, but a stopper in the overcalled suit as well.
NOTE: Some partnerships require opener to have stoppers in 1, 2, or 3 outside suits.