I'm not a Bridge player, but I've been reading about the game. This part of the rules is surprising to me. From Wikipedia's article on Bridge bidding systems,
By the rules of the game, the agreed meanings of all calls must be public and known to the opponents. In normal club or home play, the opponents are entitled, at their turn to make a call, to ask the partner of the bidder about the meaning of the call. In high-level tournaments, where screens are used, the procedure is to ask the screen-mate about their calls as well as their partner's calls. In serious online tournaments, the procedure is for the player making the call to self-alert it, but the explanation is visible only to the opponents.
Why would the meaning of a bid be public information? It's unusual and seems to take away part of the preparation that would otherwise have gone into the game. For example, if meanings were secret, then they can be pre-arranged by partners and later used to deceive their opponents. Making it all public seems to detract from the strategic depth of the game. Besides, at that point, why even bother with bidding systems? Let's say the current bid is 1 NT, and I want to know whether my partner is holding the Ace of Clubs. I could just say "I bid 2 Clubs, if you are holding the Ace of Clubs bid 2 Diamonds, otherwise bid 2 Hearts".
What is the advantage to having meanings public? The only advantage I can think of is that there isn't enough scope for strategic play otherwise, but even then this rule seems very artificial.