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.

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    \$\begingroup\$ Are you asking for an example of an addition or subtraction to damage or whether such things actually bypass immunity to a given damage type? \$\endgroup\$ – Medix2 Mar 14 at 1:31
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    \$\begingroup\$ @Medix2 If an addition or subtraction to damage does not bypass immunity, that would answer my question. Certain damage modifiers bypassing immunity seems like a counterintuitive result to me. I assumed that there existed specific, rare types of additions or subtracts (perhaps hexblade's curse) that bypassed immunity. Thanks for helping to clarify the question! \$\endgroup\$ – Joshua Mar 14 at 2:54
  • \$\begingroup\$ @Journer: As Medix2 said, if OP's misconception is the issue underlying the question, and clarifying the misconception would solve OP's problem, then you should correct that misconception in an answer (not a comment). Relevant metas: How do we handle it when the asker's problem is just that they're confused?, A question has some facts majorly wrong: should I be correcting them in comments or an answer? \$\endgroup\$ – V2Blast Mar 14 at 7:29

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.

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    \$\begingroup\$ I think the OP's question/problem is that it says to apply addition/subtraction after immunity. Seemingly, immunity would set damage to 0 and then additive damage would apply, illogically, thwarting/overcoming the immunity \$\endgroup\$ – Medix2 Mar 14 at 19:50
  • \$\begingroup\$ This might overcomplicate things but know that what happens when additive damage applies to an attack with multiple damage types is unclear. It's also difficult to define what counts as a "source of damage"; Does this mean Sneak Attack is halved separately from the rest of the attack (dealing less total damage) if a creature resists the attack's damage type (probably not)? However, the rules are applied for each damage type in an attack, even if from "a single source" (whatever that means) \$\endgroup\$ – Medix2 Mar 14 at 19:56
  • \$\begingroup\$ @medix2 like 'target', 'immunity' is undefined in the rules, so we have to look for a relevant definition, such as "not affected or influenced by something", which explains why immunity goes first, because it can cancel the other steps. \$\endgroup\$ – Journer Mar 14 at 21:16
  • \$\begingroup\$ @medix2 as for damage sources, as I recall, a source is generally the origin entity, either the creature (if no complete roll calculation is included; think pc with weapon & features), or an object (if a complete calculation is included; think siege engine or consumable item). \$\endgroup\$ – Journer Mar 14 at 21:23
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    \$\begingroup\$ @medix2 - There's no stack in 5e, so there is no after/before immunity. Immunity is applied as a steady state, it's simply a property. For instance, water is not flammable, it doesn't become flammable even if I cast Bonfire fully submerged in it. Now I just have a magical Bonfire that is heating the water, and any creature that enters the 5 foot square has resistance to fire damage from being completely submerged. \$\endgroup\$ – Lino Frank Ciaralli Mar 15 at 19:31

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.

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    \$\begingroup\$ Why should we consider step 1 to be conditional, when the rules text (and your answer) refer it to as "order" only? \$\endgroup\$ – kviiri Mar 14 at 16:54
  • \$\begingroup\$ @kviiri not sure I understand your question. Are you asking why a yes to step 1 would prevent steps 2-4? \$\endgroup\$ – Journer Mar 14 at 18:03
  • \$\begingroup\$ Yes. Why are you treating it as a conditional but not the other steps, when it's not worded any differently? \$\endgroup\$ – kviiri Mar 14 at 18:33
  • \$\begingroup\$ @kviiri it is the only step that if yes, is guaranteed to prevent the others. No other step outright prevents anything in most cases, but I will edit steps 2 & 3 for rare chance. \$\endgroup\$ – Journer Mar 14 at 19:38
  • \$\begingroup\$ But why, is the question. It doesn't say in the rules that it prevents the other steps. \$\endgroup\$ – kviiri Mar 14 at 19:58

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