826.4. It alters how players can elect to assign combat damage during combat.
826.4.a. Players must still assign lethal damage to a unit before moving to the next when assigning their damage.
826.4.b. If more than one unit with Backline is present with the same controller in Combat, damage may be assigned to any of them. Units with Backline are invalid assignments until all units without Backline have lethal damage assigned to them.