Does it apply before or after damage reduction from armor, defensive cds, and vers?
It makes a huge difference. Say if you get hit 100hp, damage reduction from all sources at 50%, and Rugged Tenacity reduce dmg flat by 1.
If you do before damage reduction then you would calculate damage reduction from (100-1)*0.5=49.5
If you do after damage reduction then you would calculate damage reduction from 100*0.5-1=49
In practice, you are getting hits in the hundreds of thousands to millions range in any meaningful PVE instance, and the flat damage reduction is 3-4k depending on your gear. It really is a moot point if it applies before other damage reduction.
In fact, if Rugged Tenacity applies before standard damage reduction, Rugged Tenacity’s flat damage reduction is reduced by the exact % of your standard damage reduction. Making it worse for tanks.