Disclosure
Declarer is entitled to know what signaling agreements you have with your partner, and you must disclose them if asked. However, you do not have to interpret any particular play (and should not, because it might transmit information to your partner). For example, if partner plays the 5 of clubs and you are asked what it means, you should simply say "a high club encourages clubs, a low club discourages clubs" (assuming that is your agreement). If you see the 4, 3, and 2 in your hand, you know that the 5 is a low club and therefore discouraging, but you should not say so. As far as your partner knows it might equally well be declarer who holds those cards and then you might misread the 5 as encouraging.
Most regulating bodies in bridge also prohibit the use of encrypted signals. These signals convey a message that can only be interpreted by knowing some specifics of the hands of the defenders. For example, declarer's bidding might promise exactly a certain number of spades. By looking at the dummy, each defender knows how many spades the other has. An agreement applied in such circumstances, such as "if I have an even number of spades at the start of play, then we play standard signals on this deal, otherwise upside-down signals" would be encrypted and therefore widely prohibited.
Read more about this topic: Signal (bridge)