Your Armor Class or AC is used to calculate the difficulty for enemies to hit you with attacks. AC is broken up into the following four different categories.
Armor Class
Stacking AC from Armor and Natural Armor
By default, AC from armor and natural armor stack, but may do so at a reduced rate. If your AC from armor is equal to or greater than your natural armor or vice versa, the lesser armor bonus to AC is treated as half rounded down. Any DR gained from the reduced armor bonus still applies fully. Additionally, if you have natural armor, you have a minimum amount of natural armor that you may benefit from equal to half of your Highest Martial Rank bonus, up to your natural armor bonus. This rule does not apply to shields.
Effective Armor Class Bonus
The highest effective ACB that you can benefit from is limited by your Highest Martial Rank bonus. For worn armor this means that you only benefits from 4 + Highest Martial Rank bonus of the armors ACB. Shields is 1 + Highest Martial Rank bonus from ACB.
This limitation applies to ACB increases from talents and materials as well. You are able to wear armor that provides more AC than your effective bonus, but do not gain the benefits of the extra AC. However, you do gain all other benefits of the armor such as DR, Material, or Property and Quality bonuses.
Armor Class (AC)
You AC is your defense against attacks from physical sources such as weapons and other similar abilities that require an attack. AC is the combined total of your Armor Bonus, Shield Bonus, Natural Armor Bonus, Dexterity Modifier, Dodge Bonus, Deflection Bonus as well as other miscellaneous modifiers, plus 10.
Flat Footed Armor Class (FFAC)
Your Flat-Footed AC protects you against the same sources as your AC, but is used Whenever you are Flat-Footed. Your FFAC is calculated the same way, but does not gain any bonuses from Dexterity or Dodge.
Touch Armor Class (TAC)
Your Touch AC is your defense against attacks from sources such as spells and other similar contact abilities that require an attack. TAC is the combined total of your Dexterity Modifier, Dodge Bonus, Deflection Bonus as well as other miscellaneous modifiers, plus 10.
Flat Footed Touch Armor Class (FFTAC)
Your Flat-Footed Touch AC protects you against the same sources as your TAC, but is used Whenever you are Flat-Footed. Your FFTAC is calculated the same way, but does not gain any bonuses from Dexterity or Dodge.
Combat Maneuver Defense
Combat Maneuver Defense (CMD)
Your Combat Maneuver Defense or CMD is your passive reactionary defense against Maneuvers and abilities that would physically move or debilitate you. Commonly used to defend against combat Maneuvers and other similar abilities. Your CMD is the combined total of your Strength modifier, Dexterity modifier, Dodge bonus, Deflection bonus, and any other modifiers from talents, XP, and other abilities.
When you are targeted by an effect that would be defended against with CMD you roll a D20 + your CMD. You are unaffected by the ability in question if your CMD result is higher than the abilities check. This happens passively and has no limitation on how many times it can occur by default.
Flat Footed Combat Maneuver Defense (FFCMD)
Your Flat-Footed Combat Maneuver Defense works in the same way as your CMD but is used Whenever you are Flat-Footed or unaware of the targeting ability. Your FFCMD is calculated the same way, but gains no bonuses from Dexterity or Dodge.
Defensive Actions
You have access to various actions that you may perform to increase your defenses. The following actions are available to all creatures by default.
Parry
You may actively attempt to deflect an enemy’s attack that you are aware of with the Parry skill by using a Defensive Action. Parrying is a defensive Reaction against an attack that you are aware of that is not an Attack of Opportunity.
If you choose to attempt to Parry, you expend a use of your Defensive Actions and roll a d20 + your bonus in the Parry skill and any situational modifiers to that skill. If your Parry check is higher than the attack roll the attack is negated.
By default, you only have 1 Defensive Action per round and are only able to Parry melee attacks made by enemies that are 1 Size Category larger than you or smaller. Your Parry attempt may gain bonuses if using a weapon or item with the appropriate properties or qualities to make the Parry attempt.
Repel
You may actively attempt to break free from a restricting effect, typically a Maneuver skill such as Grapple or other physical restrictions, by taking the Repel action using an available Defensive Action. The Repel action may be taken for free any time you would take a Basic Action, Quick Action, or Immediate Action.
When taking the Repel action you roll a d20 + your Repel skill bonus and any situational modifiers to that skill. If the check beats the DC of the restricting ability, or the opposed CMD check if actively restrained by another creature, you are freed from that effect. Repels draw from the same Reaction pool of actions as Parry does, for every Repel attempt that you make, you use a single Defensive Action.
Fighting Defensively
You may choose to fight more defensively as an altered Free Action that is part of an attack action. While taking the Fighting Defensively action you gain a 1 + half of your Rank bonus in the Acrobatics skill as a Dodge Bonus to AC, CMD, and Advantage on Parries/Repels until the start of your next turn. However you also have Disadvantage on all attack rolls while performing this action.
Total Defense
You may spend a Basic Action to focus on defense at the cost of offense. The total defense action provides you a 2 + half of your Rank bonus in the Acrobatics skill as an Insight Bonus to AC, CMD, and checks/number of Parries/Repels until the start of your next turn. However, while performing the Total Defense action you have no active threatened area, and cannot take Attacks of Opportunity or provide Flanking.
This bonus is doubled if you spend two Basic Actions to perform the Total Defense action. At base the Total Defense action cannot be performed if you took any sort of attack or Spellcasting action that turn.