Reactor 1.0 documenation has moved! See https://hanabi.wiki/e/en/conventions/reactor-1.
Reactive systems are defined by two types of clues: stable or reactive. Reactive clues are hatlike: they promise actions to players based on a hat table. Stable clues are simple: they provide information to a single player.
All reactive systems currently divide the clue space into two categories: stable clues and reactive clues.
Stable clues provide actions to only the clue receiver's hand. The exact configuration depends on the reactive system.
Reactive clues provide an action to multiple player's hands: these actions are encoded by some hat-like formulation. Additionally, the player to react is not the clue receiver.
Alice and the player reacting must agree on a card to target.
The reaction must modify the clue to provide the clue receiver an action.
Every reactive clue should mean something different in order to handle as many different gamestates as possible. If the exact card playing or discarding does not matter, this means every clue should provide the reacter a different action.
Cards that have just been drawn into the hand are more often playable or trash than cards that have been in hand and inactionable.
In order to build a reactive system, several questions require answering.
When a clue is given, the team must be able to tell whether a clue is stable or reactive.
For a system to fit in there reactive framework, there should be many hat-like clues. Configuring them is up to the individual system.
There should be clues that are relatively simple, not asking much of the team. Configuring these clues is up to the individual system.
Reactive systems may also want to answer other questions.
This question might be addressed by discussing chops and/or good touch.
Answering this question isn't specifically necessary, but systems tend to use it to help determine which clues are reactive. For example, in Reactor 0 all actions can be reactions. In Reactor 2, only discards and unexpected plays can be reactions.
Conventions are often designed with specific goals and playstyles in mind.