Do spells and abilities choose their targets when played or when they resolve?

Ruling: Spells and abilities choose their targets when they are played (added to the chain), before reactions can be made, not when they resolve. When an effect resolves, targets are checked again to ensure they are still legal, but targets cannot be redeclared.

Nuances:

• Icathian Rain is a special case because it uses reflexive triggers. The spell itself doesn't target anything when played. Instead, it creates 6 separate reflexive ability triggers that each target upon being added to the chain (not upon resolution).
• Each of Icathian Rain's 6 triggers must pay deflect separately if targeting a unit with deflect (6 power total).
• Split damage mechanics declare targets when added to the chain, but the specific amount of damage each target receives is not declared until resolution. Each declared target must receive at least 1 damage.
• With split damage, you only pay deflect once (1 power) even if dealing multiple damage to a deflect unit, because it's one instance of targeting and damage.
• Opponents cannot react to specific damage amounts from split because those amounts aren't declared until after reactions.