What counts as "any addition or subtraction to the damage" for damage immunity?

Question

According to the "Resistance and Vulnerability" section in the introduction to Xanathar's Guide to Everything (p. 5):

Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability.

Even if multiple sources give you resistance to a type of damage you’re taking, you can apply resistance to it only once. The same is true of vulnerability.

What counts as "(2) any addition or subtraction to the damage"? It seems like these additions or subtractions will bypass damage immunity.

Answer 1

Additive and Subtractive damage comes from spells, effects, features and feats, and will state it in the text.

An example of subtractive comes from the Heavy Armor Master feat, PHB pg. 167:

While you are wearing heavy armor, bludgeoning, piercing, and slashing damage that you take from non magical weapons is reduced by 3.

An example of additive comes from Great Weapon Master feat, PHB pg. 167:

Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a - 5 penalty to the attack roll. If the attack hits, you add +10 to the attack’s damage.

As far as additive damage bypassing immunity, it depends on the additive damage type. If I strike an immune to slashing target with a Flametongue longsword, it takes no damage from the weapons slashing property, however the 2d6 fire damage that it isn't immune to still burns the target.

But if you just have something like say, a Fighter's superiority die that lets you add damage, the added damage is the same as the attacks. So if a target is immune to slashing, rolling a d12 and adding 12 more slashing damage still doesn't get past the immunity. Every source of damage has to go through all 4 mechanical checks every time they're applied.

Answer 2

The lack of a Damage Immunity definition is still a problem.

If you think of it as "the propriety to set a specific damage type to 0" it will bring you some issues, like this one, but if you think of it as "the ability of ignoring a specific damage type" (wich by far i find more appropriate) that issue is gone. Here's why:

Step 1, any relevant damage immunity, is not anymore a mathemathic step, so you can apply it without reducing the damage to 0. The damage "still exists" if you wish to progress to step 2, but you take note that it will not affect the specific creature.

There is no issue with the "Specific beats Generic" rule, as Invulnerability has never been changed in steps 2, 3, 4.

There is no problem of steps 2, 3, 4 being irrelevant after step 1 is completed, as someone mentioned: it s a general rule that aims to work in all circumstances and not of being practical, wich is totally left to common sense.

There could be a problem with some effects if immunity wasn't take in consideration before the damage variations (Max Hit Points reductions, healing from hits,...).

I am aware that Xanathar's guide says "Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity..." wich suggests that damage immunity is a damage modifier, but it's not a clear definition that we can use: I think that the term refers the damage modificators of steps 2, 3 and 4, and Immunity is here just becouse it is relevant for the final outcome.

For the "multiple target problem" mentioned above, i'd say it does not affect the order of modifications we're discussing: the dices are rolled once for all the creatures affected, but after that Xanathar's suggests to follow the 4 steps once for each different creature.

There's a big "my opinion" mark on all of that, feel free to discuss with me.

Answer 3

Additions & subtractions to damage are any time when a feature, or item, or trait, allows a creature to directly increase or decrease damage.

They do not apply if the recipient of the damage has immunity (such as a door and psychic damage), as immunity outright cancels/prevents damage of a type or types. To understand this, consider the relevant definition for immune, the basis of immunity: "not affected or influenced by something." So, if something has immunity to a damage type, then it isn't affected by that type; if it isn't affected, then any changes to the quantity have no effect, so you dont consider those changes. This is why immunity consideration is the first step, because it tells you if the result is guaranteed to be zero or not.

One example of decrease is the recent Interception fighting style from from Class Feature Variants UA, which allows a PC to reduce damage to an ally (or themselves) by 1d10+proficiency bonus as a reaction.

The text you quoted from Xanathar's Guide is basically an order of operations for damage, and you can read it like this for each type of damage in an attack/saving throw (slashing, poison, etc):

1- does the creature/object have immunity to this damage type? If yes, no damage can result of this type. If no, move on to step 2.
2- does anything add to or subtract from damage of this type? Change the damage accordingly, and if any remains, then move to step 3.
3- is there resistance to this damage type from any source? If yes, reduce this damage by half (rounded down), then move to step 4 if any remains. If no, move on to step 4.
4- is there vulnerability to this damage type from any source? If yes, double this damage, and apply. If no, apply figured damage.

You follow the above steps for each type of damage that might occur, which could be 2 or even 3 separate types of damage at once. But, it is much easier when you're playing to do this, than reading it might suggest.