They're additive. One of the main reasons why they're there from a design perspective is to lessen the anti-synergy between scaling two different skills that scale with different damage types, but are thematically connected.
For example, aside from strength, the Swipe and Summon Wolf skill on Primalist do not scale with any of the same stats. For Swipe you normally want melee damage and increased physical damage, while for Summon Wolf you want increased minion damage. So, if all your increased damage were coming from sources like this and you had a total of 600% increase damage in a full build, then splitting your increased damage across stats for these two skills would result in you have ~300% increased damage for each. This means each would deal around 43% less damage than they would have if you'd focused on just scaling one of them, making this sort of build very suboptimal.
As a design philosophy, we want it to be viable to make a build that combines skills that...
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