Flat DMG x Or additive DMG?
Could you possibly give an example as to what it is you are referring to?
If the item says +100 damage, then it’s just adding a flat amount. If it says +100% it depends if there’s an [ x ] or not.
The roll on weapon % DMG is that multiplicative DMG or additive there is no x just %dmg
If you roll +DMG on a weapon it goes into the additive bucket and the total of that bucket is used as a multiplier. The additive bucket is very powerful, but it diminishes in quality as you add more multipliers from other sources.