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Last Epoch (like at least one other ARPG) is rather specific in its damage calculation rules. All applicable percentage “increases” to damage, regardless of source or type, are first added together and only then applied to multiply the damage (unless otherwise specified).
For example, if you have 100% increased elemental damage over time and 100% increased damage over time, they’re first added together, resulting in a 200% increase, and only then applied. With these numbers, this results in tripling the original damage. This type of calculation is called “additive”, to distinguish it from applying each increase separately.
Separately applied, each 100% increase would result in doubling of the damage, quadrupling the original value. This is called “multiplicative” and is generally indicated by the word “more” or otherwise explicitly specified. Multiplicative modifiers are far more powerful and are used sparingly for balance reasons; most gear affixes and passive nodes are increases rather than more multipliers.