Some abilities include an effect that uses the term “for each” to determine the magnitude of the effect. When an ability uses this term, the number of iterations of the ability is determined before any iterations are resolved. Then, each iteration of the effect is resolved fully, one iteration at a time.
If an effect would deal damage, all damage is dealt simultaneously.
Example: Shard of Pain reads “Play: Deal 1 damage to an enemy creature for each friendly Shard.” That damage may be distributed among multiple creatures, but is dealt simultaneously.
Some abilities specify that a player must “choose a creature,” then do an effect to that creature using the term “for each.” Such abilities only affect a single creature.
Example: Red Planet Ray Gun reads “This creature gains, “After Reap: Choose a creature. Deal 1 damage to that creature for each Mars creature in play.”” That damage must be dealt only to the chosen creature—it cannot be distributed among multiple creatures.