Rule 465.2.c.5
When assigning damage in this way, replacement effects that would apply to the resulting damage are considered to apply to the assignment instead. Example: A unit with 2 [M] is being assigned damage in the combat damage step. The unit has “prevent the first 3 damage I would take each combat.” The unit would need to be assigned 5 damage in order to have lethal damage assigned to it. Example: The attacking player is assigning their 3 [M] worth of damage to two defending units with 2 [M] each. One of the units has a delayed replacement effect applied to it that reads “Double all damage that would be dealt to it this turn.” The attacking player assigns two damage to the other defending unit, then when assigning damage to the unit with the delayed replacement effect they assign 2 damage to it; 1 damage that doubles to 2 damage as it is assigned to the unit. When that damage is dealt, it doesn’t get doubled again—the doubling is considered to have already happened during damage assignment. Example: The attacking player is assigning their 3 [M] worth of damage to a 2 [M] unit. That unit has a prevent value of two being applied to it, as well as the effect of Lotus Trap, doubling the damage dealt to them. Both of these replacement effects apply to the assignment of damage, in the order of the controller of the 2 [M] unit’s choice. If they choose to order the replacement effects so that the prevent value is applied first, the unit will prevent 2 of the assigned damage, then the last 1 point of damage will be doubled to 2. The unit will have 4 damage assigned to it. When damage is dealt, the unit will take 2 damage. If they choose the other order, the unit will have 6 damage assigned to it, 2 of which will be prevented. When damage is dealt, the unit will take 4 damage.