Historical Rules · 19 January 2018
Rule 810.7f
Official source text from the 19 January 2018 Rules release
As the combat damage step begins, the active team announces how each attacking creature will assign its combat damage. If an attacking creature would assign combat damage to the defending team, the active team chooses only one of the defending players for that creature to assign its combat damage to. Then the defending team announces how each blocking creature will assign its combat damage. See rule 510.1.