Stats Guide - Partial DMG

The Partial DMG(or "pdmg") stat is a damage multiplier that becomes
applicable when you miss an enemy because you lack the Accuracy to hit them. 

It could be an important stat if your lack of accuracy is an insane one (even though it is usually recommended to fix this lack instead of aiming high values for pdmg).

The pdmg are symbolized by grey numbers appearing above your head or enemies' one. To summarize roughly: getting your hits missed with the basic rate for Partial DMG corresponds to a 75% damage dealt independently from your attack variation. It means it could be a 80% * 75% damage output if you get very unlucky, which represents only 60% damage overall.
Increasing this rate, or avoiding missed hits by increasing your accuracy value, becomes quite relevant at this point.

Partial Hit Calculation formula here

At the moment, and because of the way raids and gear sets are built, running with the pdmg meta (using a very high pdmg rate and very low accuracy value) seems to be quite useful in Lunar Fall and maybe Violent Sun as well. For other raids, the min. requirement for 0% miss rate is too low to fully benefit from your pdmg rate.