Minecraft wiki entry for JSON_format, associated text:  conditions:
 signal_strength: The redstone signal that will come out of the target block.
 projectile: The projectile used to hit the target block.
 shooter: The player who shot or threw the projectile. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 player: The player that would get the advancement. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]