Rule 310.9 · Battles
Each battle has a player designated as its protector.
Subrules
As a battle enters the battlefield, its controller chooses a player to be its protector. Which players may be chosen as its protector are determined by its battle type (see rule 310.12). If it has no battle types, only its controller can be its protector.
A battle’s protector can never attack it. A battle can be attacked by any attacking player for whom its protector is a defending player. Notably, a Siege battle can be attacked by its own controller.
A battle’s protector may block creatures attacking that battle with creatures they control. Creatures controlled by other players can’t block those attackers.
If a battle’s protector is a different player than its controller, all rules and effects that refer to the “defending player” relative to a battle that is being attacked refer to that battle’s protector rather than its controller. See rule 508.5.
If a rule or effect refers to the player who protects a battle, it means the player who is that battle’s protector.
A battle can have only one protector at a time. A battle’s protector stops being its protector if another player becomes its protector.
A battle’s protector doesn’t change if it stops being a battle or it becomes a copy of another battle.