Critical Hits and Precision Damage

Definitions

Precision damage: Some forms of attack (such as that dealt by a rogue's sneak attack or a swashbuckler's precise strike) use precision damage. Precision damage suggests that the attack's effectiveness comes from the character having found a weak point in their opponent's body. Precision damage applies to more creatures than it did in previous editions of the game. Precision damage is never multiplied on a critical hit.

Critical hit: Critical hits occur when a character rolls within a weapon's critical range when making an attack roll. Critical hit damage represents an exceptional hit. The process for a critical hit is:

1. Roll your attack. If the unmodified d20 result is within your weapons threat range (expressed often as 18-20 or 20), proceed to step 2.
2. Roll the attack again. If it hits, the critical hit is confirmed.
3. Roll a number of damage dice equal to your weapon's critical hit modifier. In addition, multiply all non-precision, and non-added dice (such as from frost) by this amount. For example, on a x3 weapon such as a greataxe, your 1d12+8 damage becomes 3d12+24.

What's Immune to What?

Critical Hits

The following have immunity to critical hits. That is, instead of multiplying, you will roll damage as normal:

Aeon (subtype): "Immunity to cold, poison, and critical hits."

Elemental (subtype): Elementals are "not subject to critical hits or flanking. Does not take additional damage from precision-based attacks, such as sneak attack."

Incorporeal (subtype): "An incorporeal creature is immune to critical hits (unless the attacks are made using a weapon with the ghost touch special weapon quality.)"

Ooze (Type): Oozes are "not subject to critical hits or flanking. Does not take additional damage from precision-based attacks (such as sneak attack.)"

Protean (subtype): Proteans posses a 50% chance to ignore critical hits. This is due to their ability "Amorphous Anatomy" which might protect them: Amorphous Anatomy "grants a 50% chance to ignore additional damage caused by critical hits and sneak attacks."

Swarm (Type): "A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits."

Precision-Based Damage

The following do not take additional damage from precision-based attacks:

Elemental (subtype): "<An elemental...> does not take additional damage from precision-based attacks (such as sneak attack.)"

Incorporeal (subtype): "An incorporeal creature is immune to precision-based damage (such as sneak attack damage) unless the attacks are made using a weapon with the ghost touch special weapon quality."

Ooze (Type): "<An ooze is...> does not take additional damage from precision-based attacks (such as sneak attack.)"

Protean (subtype): (50% chance to ignore, see below*)

Creatures Immune to Flanking

Flanking

Opponents do not gain any special flanking bonuses against the following:

Ooze (Type): "<An ooze is...> not subject to ... flanking."

Swarm (Type): "A swarm has no clear front or back and no discernible anatomy, so it is not subject to flanking."

Elemental (subtype): "<Elementals are...> not subject to flanking."

Material adapted from PDSRD.com.