Historical Rules · 1 October 2009
Rule 509.3
Official source text from the 1 October 2009 Rules release
Third, for each creature that's blocking multiple creatures (because some effect allows it to), the defending player announces its damage assignment order among the attacking creatures. This turn-based action doesn't use the stack. (During the combat damage step, a blocking creature can't assign combat damage to a creature it's blocking unless each creature ahead of that blocked creature in its order is assigned lethal damage.)