Aside from ability scores, all creatures have a set of statistics that represent their basic ability to deal and evade damage. These numbers increase as creatures increase in power through gaining levels. The secondary statistics are Armor Class, Hit Points, Base Attack Bonus, and Fortitude, Reflex, and Will base saving throw bonuses. All creatures also have some capacity of noticing things others don’t want them to, which is represented by the Awareness defense.
Armor Class (AC)
Armor Class measures a character’s skill at evading physical attacks. It is closely linked to the Base Attack Bonus (BAB). You calculate AC as follows: 10 + your character level + your Key Defense Modifier + any other applicable modifiers. You only use the first number in any BAB entry when calculating your Armor Class.
Hit points (HP)
Hit points measure a character’s overall combat endurance. Your HP largely depend on your class; each class offers 8, 10, or 12 hit points, plus your KDM, per level, with twice as many HP at 1st level. For example, a 1st-level barbarian with Constitution 16 would have 26 HP (10 class HP, plus a Constitution KDM of 3, doubled for 1st level). At 2nd level, that same barbarian would have 39 HP.
Base Attack Bonus (BAB)
Base Attack Bonus measures a character’s basic skill at combat. It increases at one of two different rates, depending on a character’s class, as detailed on the table "Base Attack Bonus." You use BAB to calculate an attack roll against an enemy, as follows: 1d20 + BAB + your character’s Key Offensive Modifier + any other applicable modifiers. Whenever you make an attack roll, if the result equals or exceeds the target’s Armor Class, the attack hits that target.
When they take an attack action, characters with a BAB of +6 or higher gain one or more extra attacks, albeit made with a lower attack bonus, as shown on the "Base Attack Bonus" table.
Awareness is your character’s general ability to notice when someone else is trying to perform an action while remaining unnoticed. You calculate Awareness as follows: 10 + your level + your character’s Wisdom modifier + any other applicable modifiers.
Saving throw bonuses are used to calculate “saving throws” or "saves," which are d20 rolls made to resist particular types of abilities. Abilities that require a save specify the type of save required, but a general guide for what saves resist which types of effect is given below. You can make a saving throw against an effect whenever it would specifically allow you to, or whenever that effect would affect you differently on a successful save. Unless otherwise specified, you can only make a single save against each individual effect. Saving throw bonuses are based on a character’s level and choice of class, and are given in the table for a character’s chosen class. A save succeeds if its result equals or exceeds a listed Difficulty Class (DC), which is always given in the text of the ability or effect that triggers the save. You may always decline to make a save against an effect that triggers a save. This is treated as failing the save for that effect.
Fortitude is a character’s ability to resist effects that eat away at that character’s physical body or crush it outright, such as poison or a cartoonishly impractical anvil. You calculate Fortitude as follows: 1d20 + your base Fortitude bonus + the higher of your Strength and Constitution modifiers + any other applicable modifiers.
Reflex is a character’s ability to dodge area effects, such as explosions, as well as most traps. You calculate Reflex as follows: 1d20 + your base Reflex bonus + the higher of your Dexterity and Intelligence modifiers + any other applicable modifiers.
Will is a character’s ability to resist assaults on the mind, such as scary things or mind control. You calculate Will as follows: 1d20 + your base Will bonus + the higher of your Wisdom and Charisma modifiers + any other applicable modifiers.