Can someone explain to me how %increased elemental damage on weapons work?

I usually can understand the Math of most of the game alright, but i cant with this. What it boosts? Only the local damage of the weapon? Conversion too? If it was the case shouldnt it boost avatar of fire damage greatly? What i see isnt the case, when i use avatar of fire a good phys weapon always outclasses the modifier, but for something like artillery ballista it boosts it greatly. I would be very thankfull if someone could explain it to me.