Historical Rules · 1 October 2009
Rule 509.2
Official source text from the 1 October 2009 Rules release
Second, for each attacking creature that's become blocked by multiple creatures, the active player announces its damage assignment order among the blocking creatures. This turn-based action doesn't use the stack. (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.) 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.