With thanks to TheSymbol for information on how the UI mod calculates this.
What is Coefficient?
Coefficient is a calculated stat from players in order to help them determine their combat strength by combining Damage, Critical Damage and Critical Hit.
You can get the mod to display this in-game from TheSymbol here: Damage Coefficient Stat
How is it calculated?
((Damage * (Critical Damage / 100)) * (Critical Hit / 100)) + Damage