What is the difference of additive and multiplicative damage?

Things with the same stat bonus add together, while different stats tend to multiply together.

For example, pressure point and condition overload add their bonuses together because they are both adding to the same damage stat. If you have pressure point with +120% damage, and CO giving say +360% damage because there are 4 status effects, you add that damage together to get your final damage, +480% damage, and that's additive stacking.

But say you have berserker fury and CO instead. Now you have berserker fury giving you +70% attack speed, and CO giving +360% damage. These synergize together and effectively multiply each other's damage instead of simply adding together. Giving you +682% damage between those two mods instead.

And faction mods like smite grineer are considered a seperate stat than raw +damage, so they end up multiplying together with raw damage. But multiple sources of facfion damage, such as a riven and a mod, would add together as well.

First thing to note is there are different kinds of damage.

The same kinds of damage are additive with each other.

Different kinds of damage multiply with each other.

For exampe:

Serration counts as base damage. An ability like Chroma's 3 counts as base damage. These are additive with each other.

Faction damage is another kind of damage. Bane of Grineer is a mod that counts as faction damage. An ability like Rhino's Roar counts as faction damage. These are additive with each other.

Base damage and faction damage are multiplicative with each other since they are different kinds of damage.

Here is the formula for the example:

total multiplier = (1 + Serration + Chroma)*(1 + Bane of grineer + Roar)

Hornet Strike + Magnum Force + Augur Pact is just adding more base damage on top of itself.

An elemental mod multiplies the base damage instead. Then Multishot leads to multiplying all that. And a critical hit is a multiplier to all that. And Fire Rate is a multiplier to all that. And faction mods are a multiplier to all that.

When you stack multipliers it gets absurd. I have a pistol that can do 130k dps. It uses all those multipliers.

https://warframe.fandom.com/wiki/Calculating_Bonuses has details on this plus what players refer to as “diminishing returns”.

additive damage means you are boosting your damage stat where multiplicative damage means you are multiple the total damage you have showing, it's like saying you deal 100 damage and you are trying to do multiplicative damage so you need crit damage which multiplies from 100 and makes it go to 1000 whereas if you want additive damage you go for just outright damage such as pressure point so you go from 100 to 105 or something like that if that makes sense