Some abilities include an effect that uses the term “for each” to determine the magnitude of the effect. Unless otherwise specified, a player may choose to affect a different card with each instance of such an effect.
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.
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, “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.