Magical Resistance Rating

Overview
Magical Resistance Rating (abbreviated MRR) is a form of damage mitigation was that introduced with Update 23.

It provides a percentage reduction in damage from acid, sonic, fire, electric, cold, alignment, light, positive, negative, poison, and rust. MMR is granted through multiple sources (items, feats, enhancements) and stacks with itself. Multiple effects granting an Enhancement bonus to Magical Resistance do not stack. Character sheets show the Magical Resistance Rating of the player next to the elemental resistances.

MRR is applied after resistances. MRR is applied to the damage that passes through the resistance and this is always rounded down. As such if a player would take 20 damage before resists or MRR was applied and had 10 resistance and 60 MRR (62.5% reduction), the result is:
 * (20 - 10) * 37.5% = 3.75 round down to 3
 * 3 damage will be resisted.

Formula
100/(100+Rating)

Example: A character is hit for 100 points of damage.
 * 0 MRR takes full damage, 100 points.
 * 20 MRR takes 16.67% less damage, 83 points.
 * 100 MRR takes 50% less damage, 50 points.

MRR Cap
Different armor types can apply a cap for Magical Resistance Rating:
 * Robes or Outfit: 100
 * Light Armor: 200
 * Medium Armor: No cap
 * Heavy Armor: No cap

Heavy and tower shields also prevent you from using Evasion.

Items
Currently armor is only giving 10/20/30 for light/medium/heavy
 * Magical Resistance Rating offered by armor type per U25 release notes
 * Robes or Outfits: No MRR
 * Light Armor: 10 MRR +2 +1/2 BaB
 * Medium Armor: 20 MRR +4 +2/3 BaB
 * Heavy Armor: 30 MRR +6 +BaB
 * Magical Resistance Ratings offered by shield type:
 * Buckler: No MRR
 * Light Shield: 5 MRR
 * Heavy Shield: 10 MRR
 * Tower Shield: 15 MRR
 * Enhancement bonus: Sheltering items (except Insightful Sheltering items)
 * Insight bonus: Magical Resistance items and Insightful Sheltering items

In addition, characters who are proficient with shields can also use the larger shields for defense against magical attacks that would normally require a Reflex saving throw. Instead of (or in addition to) using your Reflex saving throw to mitigate the damage you can deflect the damage off your shield. This is represented by increasing your Physical and Magical Resist Ratings against those types of attacks when using a shield. Note that you gain this additional mitigation whether you make your Reflex saving throw or not.

Physical and Magical Resist Rating multiplier against magical attacks that normally allow a Reflex saving throw.
 * Buckler: 1.0 (no additional mitigation)
 * Light Shield: 1.0 (no additional mitigation)
 * Heavy Shield: 2.0 (no evasion)
 * Tower Shield: 2.0 (no evasion)