Rule 509.2

Second, for each attacking creature that's become blocked, the active player announces that creature's damage assignment order, which consists of the creatures blocking it in an order of that player's choice. (During the combat damage step, an attacking creature can't assign combat damage to a creature that's blocking it unless each creature ahead of that blocking creature in its order is assigned lethal damage.) This turn-based action doesn't use the stack. Example: Craw Wurm is blocked by Llanowar Elves, Runeclaw Bear, and Serra Angel. The Craw Wurm's controller announces the Craw Wurm's damage assignment order as Serra Angel, then Runeclaw Bear, then Llanowar Elves.

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