Explain the wording that makes Cull the Weak is not choosing on activation but Beast Below is.

The difference lies in how the instructions are phrased and whether they are categorized as targeting. In Riftbound, a spell or ability must "target" any object it selects if that selection is made when the card is played (finalized on the chain). If the selection is made during resolution, it is not considered a target.

Cull the Weak: Resolution Choice

• The Text: "Each player kills one of their units."
• Why it isn't choosing on activation: According to Rule 355.10.e, "Each player kills a unit they control" does not target. Because the effect instructs players to make the choice during resolution, no targeting takes place when you play the card.
• The Result: You do not have to identify which unit will be killed when you place Cull the Weak on the chain. In fact, if you have no units when you play it, you can still play the card, and you simply "do as much as you can" when it resolves (FAQ #8580).

Beast Below: Activation Target

• The Text: "Return another friendly unit and an enemy unit to their owners' hands."
• Why it IS choosing on activation: Unlike Cull the Weak, Beast Below uses language that requires you to identify specific game objects as part of the ability's requirement.
• Targeting Rules: According to Rule 355.10, any game object mentioned in the text is a target unless specific exceptions apply (such as it being in a non-public zone or part of a cost). Since Beast Below is not a "kill all" or "each player" effect, but rather an effect that selects specific units to bounce, those units are targets.
• The Result: Because these are targets, you must have valid targets for both a "friendly unit" and an "enemy unit" to place the triggered ability on the chain (FAQ #466). If there is no valid enemy or friendly unit to target, you cannot put the triggered ability on the chain at all, although you are still allowed to play the Beast Below unit itself.

Summary Table

In short: "Each player chooses" is a resolution-based instruction that does not target, whereas "Return [x] and [y]" identifies specific units as targets that must be locked in when the ability is placed on the chain.