A player's AP, or Armor Points, is a gauge of how much armored protection they have left.
AP will "absorb" damage from bullets and projectiles, but only up to a certain amount. The amount of damage reduction is also dependant on the player's Vest; Light Vest-users (who have less protection) will have a minimal amount of damage transferred to AP, Medium Vest-users (with a standard amount of protection) will have a decent amount transferred, and Heavy Vest-users (with a large amount of protection) will have great amount transferred, and so on.
Basically, a better vest equals a higher chance of "tanking" a shot. On the other hand, protective vests often encumber you and greatly deplete your speed and mobility.
Unfortunately, AP isn't as effective in Combat Arms as it should be. Though it supposedly reduces damage, the actual amount is very minimal compared to that of certain vests, or even in real life. Most of the time, damage will still "penetrate" your armor and kill you regardless, as AP offers practically NO damage resistance to explosives and melee, no protection from head or backshots, and a very slight damage reduction against bullets. More often than not, when you die, you will still have a lot of AP remaining (because it practically did nothing).
A "glimpse" of true AP can be seen when playing as a Spy in Spy Hunt, or an Infected in Quarantine. Because your life is actually dependant on your AP, it really does have its value in those particular game modes.
The Super Spy in Spy Hunt has an astounding 500 AP, which can protect from explosives, melee, and fire. It also greatly reduces bullet damage, making it about less than 1 HP per bullet. This is the type of AP that most players expect when they begin to play Combat Arms.
The Infected in Quarantine are heavily dependent on AP. Their HP will vary with the type of vest that they wear, though their speed will remain exceedingly fast. This can be a 50/50 situation for most players, as most humans usually rely on speed to quickly evade attacks and to dodge around. However, they will have very low HP as an Infected, a major disadvantage. On the other hand, having a powerful vest allows massive amounts of HP when Infected. This can be vital against any survivors with explosives or other powerful weapons. The cost? They can barely move when they are still "alive", rendering them quick and easy victims.
- Take note that the AP will always remain 100 and cannot be depleted as infected.
It is widely regarded that AP doesn't work in Black Lung or Cabin Fever. The Infected's melee attacks (as stated, Melee isn't affected by AP) simply bypass whatever armor you have on, and it's usually better to be mobile than to be encumbered by unneccessary weight. In Desert Thunder and Desert Fox, your vest works better, but it's almost just as useless.
Combat Arms:Zombies (iOS)Edit
While in Combat Arms online, AP does nothing but stay as a placeholder for Nexon, in the iOS version however, almost every item has AP points that grants protection. Until all your AP is gone, you don't take serious damage.