Historical Rules · 3 May 2019
Rule 509.3
Official source text from the 3 May 2019 Rules release
Third, for each blocking creature, the defending player announces that creature’s damage assignment order, which consists of the creatures it’s blocking in an order of that player’s choice. (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.) This turn-based action doesn’t use the stack.