"There are situations," said Spock, "where you may want to make exactly the same bid, but each time you want that bid to have different meanings."
"But this cannot be possible," said the Romulan, "It is , as you would say - illogical."
"Precisely so. But let us look at a possible scenario. Partner deals and opens 1NT (12 - 14) and the opponent on your right (RHO) makes an overcall of 2S. (5+ Spades. Less than 15 HCP). you hold this:"
"You want to be in game," said Spock, "but which one? So what do you bid?
"Um, 3N, 4H or 5C all look possible contracts," said the Romulan. "So I must bid a game forcing 3C and see what partner does."
"Exactly so. But what if you hold a hand like this?"
"Now" said Spock, "You do NOT want to be in game. On the other hand you don't want to sell out to a measly 2S do you?"
"No way Jose!" thundered the Romulan. "So I bid a non-forcing 3C..... Er, uh-oh. Molto Problemo, huh?"
"Oh yes indeed. How is partner supposed to be able tell when you bid 3C that it's either (1) forcing to game, or (2) just a mild competitive bid with no further ambition?
The Romulan frowned. "I hope partner is psychic? Or maybe I cough when I am weak?"
"That is called cheating and I hope you never consider such a thing. Also, partner is never psychic and can be guaranteed to get it wrong. However there is a little wrinkle that can iron out these situations. What you can do is use the bid of 2NT as a TRANSFER to Clubs when you are weak, and a direct bid of 3C when you are strong. This leaves you with DOUBLE for the invitational hand that would have bid 2NT without the overcall. Be aware that this is NON - standard and must be agreed with partner before the game commences."
You can use this with any suit, but make sure you discuss it beforehand!