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pdr - integer physical damage reduced. No cap.
%dr - percentage physical damage reduced. Capped at 50%
mdr - integer magical damage reduced. Applies to magic/lightning/fire/cold damage.
%mdr - resists. You all know how resists work. Capped at 75% unless you got gear increasing maximum resists.
Absorb - integer absorb. No cap. Magical absorb only affects magical damage, not elemental damage.
%Absorb - percentage absorb. Capped at 40%. Magical absorb only affects magical damage.

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The damage dealt is reduced by above mentioned 'modifiers' in a specific order. The order is as follows:
1) XvX penalty (PvP penalty = -5/6)
2) Sorceress Energy Shield
3) Necromancer Bone Armor and/or Druid Cyclone Armor
4) Integer DR/MDR
5) Percentage resists (including DR%)
6) Percentage absorb
7) Integer absorb

DR%: 50% is the max. Everything above that is ignored until you are amped. If you have, let's say, stacked 80% DR and a level 1 amp is cast on you, you will still have 50%.

DR/MDR has no limit. Iron Skin's DR is directly added to your total DR, so it is applied at the same time.

With a minute spent on googling, I found the following:

Full topic here

dr% is the same as elemental resists but for physical damage, it is important yes

It see's you through all of NM too if you focus on it. I'm still negating pretty much everything in act 5 nm including chargers with 140ish PDR/MDR and diablo dealt very little damage (could get by with super heals)) since its applied before amp, it helps alot. I'm easily out tanking most of the barbs around the same level and I only got 3k life pre buffs. I'ma stick with pdr/mdr till she dies to see how far it can take you. (only 8 levels till I push her to over 200 PDR/MDR baby!)