Title.
Ex.

// spawn.json
[
  {
    "biome": "Plains",
    "function": "say hi"
  }
]

Run "say hi" when a mob spawns in a Plains biome. In this case the function would be run as the mob being spawned and at it's location, but for other rules this could be different, like for example, rules in rightclicks.json could be run as the player and at the block being right clicked or something similar.