Damage reduction (or soak) represents a creature’s ability to ignore a certain amount of physical damage from a single hit. Non-physical damage, such as from elemental or magical attacks, is unaffected by damage reduction.
Damage reduction is typically expressed in the form “20/+3” or “+3 soak 20 damage”. This indicates the amount of damage ignored (20) and the power of the weapon needed to penetrate the effect (+3). Thus a creature with 20/+3 damage reduction ignores the first 20 points of damage from any weapon that does not have an enhancement bonus or attack bonus of +3 or greater. Some feat-based damage reduction is expressed in the form “20/-“. In this case, all physical damage is reduced, regardless of bonuses on the weapon.
Creature weapons receive special treatment when trying to overcome damage reduction. A creature with damage reduction who uses creature weapons is able to pierce (ignore) damage reduction of the same power level as its own. For example, the claw attacks of a creature with 10/+5 damage reduction will not be affected by others’ damage reduction of X/+5 or lower. A creature’s X/- damage reduction (as from feats) does not assist in overcoming damage reduction.
Damage reduction stacks with, and is applied after, both damage resistance and damage immunity. (So a creature with damage reduction 10/+5 and bludgeoning resistance 20/- will ignore the first 30 points of damage from each bludgeoning attack made with less than a +5 enhancement.) Damage reduction does not stack with itself, with a few exceptions. Instead, when a creature has multiple sources of damage reduction, the one that reduces the most damage among those that are not overcome by an attack’s enhancement bonus will apply. If there is a tie, the oldest reduction effect is used. One exception to the stacking rule is that the epic damage reduction feats stack with perfect self. The other exception is that the sources of X/- damage reduction (barbarian-, dwarven defender- and epic- damage reduction) stack with each other.
Damage reduction is an ability of the barbarian, monk and dwarven defender classes, and some feats and spells provide it as well. Some druid/shifter forms also include the effect.