"damage modifiers" refer to factors that change or affect the amount of damage caused or received in a certain situation. It could be something that increases or decreases the damage inflicted. Full definition
There is supposed to be balancing in all normal modes that takes away specific damage modifiers for weapons allowing everyone to be on a even playing field, regardless of level and gear. (gamegrin.com)
Other errors that will be fixed by the patch include sudden game crashes, collision problems, map markers not showing up, fall damage modifier, and unable to earn BP. (techtimes.com)
Once the Fury's encounter has been cleared, the weapon will affect the character that wears them with damage modifiers for certain elements, dungeon modifiers, but also, they do have a negative effect sometimes. (blastawaythegamereview.com)