How do you know if a spell or ability targets? Specifically, why does Cull the Weak not require announcing targets while Vengeance does?

Ruling: A spell targets unless an exception applies. Cull the Weak doesn't target because the choice of which unit to kill is made in part by other players (each player chooses one of their own units), which is an exception to the targeting rule. Vengeance targets because one player makes the entire choice alone.

Sequence:

• When casting a spell that targets, targets must be announced during finalization
• When casting Cull the Weak, no targets are announced during finalization
• During resolution of Cull the Weak, each player then chooses one of their units to kill

Nuances:

• The fact that Cull the Weak says "player" in the text is not the reason it doesn't target - it's specifically because the choice is distributed among multiple players
• In both cases (Cull the Weak and Vengeance), the spell itself is responsible for killing the unit, not the player making the choice
• A spell that said "the player controlling this chooses a unit and kills it" would still target because one player makes the entire choice