Is not described in the official Grim Dawn Game Guide, and I can’t seem to find any succinct explanations of what exactly it is and does elsewhere to boot, so yeah I’m requesting it be added, most likely in the Combat subsection of Gameplay where the other mechanics are described.
It does what it says. It reduces reflected damage.
It’s mainly useful for Physical damage dealers that have low Armour/Physical resistance to take their own hits reflected back at them, especially in the case of Internal Trauma as this bypasses Armour so is subject to how high you can get your Physical resistance and Reflected damage reduction else you melt yourself to death haphazardly 
High DPS glass cannons or big DoT dealers (such as Bleed) builds will also feel their damage reflected damage back at them so it can be nice to have on them as well if you happen to pick it up alongside other stats.
Is it additive or multiplicative?
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