Historical Rules · 4 October 2019
Rule 509.6
Official source text from the 4 October 2019 Rules release
If a spell or ability causes a creature on the battlefield to block an attacking creature, the active player announces the blocking creature’s placement in the attacking creature’s damage assignment order. The relative order among the remaining blocking creatures is unchanged. Then the defending player announces the attacking creature’s placement in the blocking creature’s damage assignment order. The relative order among the remaining attacking creatures is unchanged. This is done as part of the blocking effect.