Can explain %attack mutiplicative and %total/boss damage additive?

Easiest explanation I can come up with. If you have absolutely nothing, you are dealing 100% damage.

Let's pretend you have some separate gear with 20% boss, 30% boss, 5% attack, and 5% attack.

ex1:
If you have two sources of %boss, they will add to each other.
100%*(20% boss+30% boss) = 150% damage on bosses.

ex2:
If you have two sources of boss, and two sources of %att, they will only add to their own kind... then multiply each other... easily seen in:

100%*(20% boss+30% boss)*(5% attack+5%attack)=100%*150%*110%=165% damage on bosses

the wrong way:
100%(20% boss+30% boss +5%attack+5%attack)=160% damage on bosses (once again this is the WRONG way)




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To further explain tecul1's post....

two pieces of gear with 20% damage:
100%*(20%+20%)= 140%

(which is the same as 1 piece with 40% damage)

But if you have two pieces of gear with one at 20% damage and one with 20% attack:
100%*(20% dmg)*(20% attack)= 144% damage

The higher the numbers get, the more noticeable the differences will become.





TL;DR (overly simplified but still somewhat accurate) formula

100%* (Sum of all boss% and dmg% boosts) *( sum of all %att boosts)