Provided that we both understand this, you will not leave me to play a contract of one club, but will make some other bid, natural or artificial. The main restriction on agreements between partners about the meaning of bids is that all such agreements must be declared to the opponents. A bidding system is a comprehensive set of partnership agreements about the meanings of bids. In addition, any player, at their turn to bid or at the end of the auction, may ask for and be given an explanation of the opponents' bidding agreements. The explanation should be given by the partner of the player who made the bid in question.
Similar considerations apply to the play.
Partners may agree on the meaning of the choice of card played in certain circumstances. Again, the opponents are entitled to know about such agreements. They may be asked about during the play. In rubber bridge one does not often come across complicated systems and partnership agreements. One is often playing with an unfamiliar partner, or in an informal setting.