What type of rule is the Red Rule, and where does it come from?

Question

Exalted has, in its third iteration, "the Red Rule".

The Red Rule

In almost all aspects, Exalted doesn’t mechanically distinguish between Storyteller characters and those the players control. Here’s the exception:

A player-controlled character can only be seduced or otherwise put in a sexual situation if the player is okay with it. Otherwise, any such attempt fails automatically.

This is completely up to the player’s discretion, and they can waive this rule’s protection if they want their character to be seduced, if they think it would improve the story, or for whatever other reason. This is entirely up to the player, and on an attempt-by-attempt basis—waiving the rule once doesn’t void your ability to call on it later against the same character, or even in the same scene. If no one in your group ever invokes this rule, that’s also fine—but players don’t have to watch their character put into a sexual situation they’re not comfortable with.

Groups that feel comfortable in doing so should allow player characters the full range of their seductive prowess when entangled with Storyteller-controlled characters… although remember that the Storyteller is also a player and their boundaries should be respected.^{Exalted 3 Core, p222}

This kind of rule is pretty much a rule that is put into the baseline table contract to play Exalted. But it is based in that nobody shall be forced to endure something they don't enjoy. And this concept was formulated somewhere else, with some other term. It had to do something with a card, but... I don't remember.

Answer 1

You might be thinking of the X-Card by John Stavropoulos.

The short version is that you give your players index cards marked with a big X. These are a clear and non-verbal (so as not to interrupt the flow of play) way to signal to the GM and/or the other players that whatever is happening right now is making that player uncomfortable. This applies to any kind of content, not just sexual. Crucially, when you use the X card, you don't have to explain why. The game is adjusted and moves on.

You can also add the opposite card (an O card, or a green card, or whatever you want) for your players to signal that whatever is going on right now is exactly the kind of thing they want and you should pursue it more and/or further and/or longer.