Critical
A critical is a term that refers to the extra damage done to a creature or character in an attack. For example, a hit that does 18 raw damage to a creature wearing soft leather armor will suffer a level 3 critical, on average. The critical determines the message that is seen after an attack, the stun, the wound, how badly the creature bleeds, and the extra damage done by the critical hit.
Critical hits are determined by the raw damage and the critical divisor of the armor the defender is wearing. The raw damage is determined by the damage factor of the weapon used against the armor the defender wears and the end roll of the attack. The effectiveness of a given critical hit is different depending on which location is hit, which is normally determined randomly unless the AMBUSH verb is used, generally. Also, there are different types of critical hits for each location, which include crushing, piercing, or slashing damage. What type of damage a critical hit does is randomly determined between the possible damage types a given weapon is capable of, unless the Combat Maneuver, Precision is used. However, both the hit location and the type of damage cannot be influenced at once. If the AMBUSH verb is used, one cannot benefit from Precision, and if Precision is used, one cannot aim an attack through AMBUSH. In such a case, using a weapon that only deals certain types of damage is paramount.
Example
The italicized line below is the line known as the critical.
You swing a gleaming vultite battlesword at a thunder troll!
AS: +195 vs DS: +49 with AvD: +42 + d100 roll: +4 = +192
... and hit for 53 points of damage!
Strong slash to the thunder troll's right hand cuts deep.
The thunder troll is stunned!
Roundtime: 5 sec.
Note that the critical location was determined to be the right hand and that the damage type shown is slashing. The damage factor of a two-handed sword against a thunder troll's armor is .500, so the +192 endroll does 46 raw damage. The message given is a level 4 critical. The critical multiplier against the hand is 5, so the expected critical level is 9, but the troll is protected by natural critical padding and critical randomization reduced the critical level to 4. A level 4 slashing crit to the hand does 3 extra damage (not the above observed 7). Clearly, there's an irregularity, somewhere.