Armor Save

Armour Saves are a mechanic from both Warhammer Fantasy and 40K, and represents the model in question's armor, as well as the durability of it. In both games, armor comes in many shades and sizes - Most standard troop armor ranges from a patch of metal on the shoulder to small vehicles, but though the function and look of the armor in question can vary, they function the same in game and are performed the same. Each model has exactly one armor save, and if it wishes to defend itself against a wound, armor saves are incredibly handy—mostly because most dudes have one. Cover Saves are only gained in cover or when behind someone else, and in those cases, they are still pretty weak stuff, and Invulnerable Saves are even rarer, if better, as they can't be bypassed by Armor Penetration.

When throwing an Armor Save, you roll a single D6 per model - If the roll exceeds the Armor Save of the model in question, the wound is discarded with no further effect, but if the roll is below, the armor failed to deflect the attack and the model takes the wound. Therefore, the lower the save, the more reliable it is.

That said, there are vastly different systems for how armor works in the two systems.

Warhammer Fantasy

In ages past, the writers at Games Workshop decided that instead of a nice traditional armour save, Fantasy both systems used an extremely in-depth system of modifiers, increasing and decreasing values and other such things. Where 40k's third edition got rid of that early for a much less bookkeeping-intensive approach of 'fixed value that can be negated altogether' (Amusingly enough, eight edition is reintroducing them; see below), Fantasy kept the intricate system of save modifier for a much longer time.

Fuck those guys.

So, from the top, armour saves in Warhammer Fantasy work differently to those of 40k. Instead of a flat value, all forms of armour in the game increase the unit's armour value. So for example, an Empire Spearman has light armour, which gives him a 6+ armour save. Simple right? Well now you gave him a shield, for some reason since he can't use it in close combat with his spear, which gives him +1 to his armour save as well, meaning he now has a 5+ save, as well as a 6+ parry save in close combat (which again he can't use with his spear you fucking simpleton). Every form of armour in the game works similarly, even the really strange stuff like Scaly Skin (mainly seen on the Lustrian Reptile Regiment but sometimes on other people) and stuff like Chaos Armour, unique to Warriors of Chaos and bestowing a 4+ save on your common Warriors, becoming Space Marine levels of bullet-spongy when given a shield as well. Being mounted also gives you +1, and then horse armour adds a whole other side to the armour calculations.

Now comes the fun part.

Y'see, the problem with this system is that light armour is fucking useless and mounted heavy-armoured burly bastards are broken beyond belief, since they'll get a 2+ save for almost no points on top of riding a motherfucking Daemon horse. So the writers at GW decided that life was too simple for us Fantasy peasants, and introduced armour-affecting attacks. So let's take our old friend the Empire Spearman. He looks too damn smug with his 6+ save, poncing around the place looking at his reflection in his speartip. Let's get an Ogre to smack some fucking sense into him. *BLAM* Thank you (Ogres hit roughly as hard as a bolt pistol to the head). Now the Ogre is a solid S4, which means that he reduces the armour save of his opponent by 1 when he wounds. so the Spearmen now has a 7+ save. Whoops! That doesn't exist, so he has no save and is now fucking paste. But wait! He has a shield! That means the -1 modifier from the Ogre's attack makes his 5+ save a 6+! Hooray! Oh shit here comes an Ogre Tyrant *BLAM* who is S5, which reduces armour saves by 2. 5+2 (yeah reduce is add and add is reduce, deal with it this isn't fucking 40k anymore) equals 7 so there's no saving this Spearman. Wait! (again), his shield gives him a 6+ parry save, which is a ward save for combat, so he still has a very small chance! Whoops, he failed and died, ah well, better luck next time.

So the basic function of the negative modifier system is to ensure that heavy attacks are actually useful and feel like something happened when you use them, and also to make sure strong armour saves aren't complete bullshit. Because let's be fair, a mounted heavy-armoured shielded Knight ain't surviving shit when he's taking a cannonball to the gut. As the Strength of the attack increases, the armour save negative modifier also increases, so at S4 you reduce 1 from the save and at S10 you remove 7. This means that 1+ saves are actually legal in Fantasy, since reducing the defense they afford means they are breakable, and the "always fails on 1" rule still applies. There are also attacks that have extra armour piercing rules, like Skaven Warpfire Throwers that reduce the armour save by 2 as well as being S5, giving a healthy -4 to the save overall.

This system is often infuriating, but also helps make the game feel more authentic, and is genuinely fun at times.

Warhammer 40K

The Armor Save at first worked much the same in 40k (Albeit with its own brand of lulz. 2+ save on 2d6 for Khornate Terminators, anyone?) 3rd edition introduced a much more streamlined system. You see, 40K has armor saves flying out the arse - pretty much everyone has some kind of armor (Unlike Fantasy where your troops generally are lucky to have any sort of armor at all). Horde armies like Orks and Tyranids tend to have very low armor saves though, while more elite armies like Space Marines tend to have much better armor.

If you are new and just got your first Space Marine Codex, you might marvel over how your troops only die on 1's and 2's, believing that your new superhuman warrior models can wade out in a firefight with impunity... And you would be right. If it wasn't for every single fucking thing in the game sporting multiple weapons who laughs at armor saves without any sort of trade off to the weapon or the bearer. See, where higher strength weaponry decreases the effectivness of armor saves in Fantasy, they are simply ignored through a special stat called AP that all weapons have. The number specifies how strong an armor the weapon will ignore, which means if a weapon like the Bolter has an AP of 5, it can completely ignore every armor save of both 6+ and 5+, completely negating the need for these fuckin' saves. Seriously, if a weapon doesn't have any kind AP it is deemed weak unless it spits out a large amount of decently strong (or rending) dakka. Do note that this is one often overlooked weakness of armor : It will statistically fail. No matter how lowly a weapon is, no matter how ridiculous or nonsensical it looks that it may get through your precious armor save, it will. This is why while you may think that huge blob of lasguns won't do jack to your terminators, it may just as well, with a bit of luck, a good order and the help of the rapid fire rule, wipe it.

So, armor in 40K is basically just a suggestion. No matter what, the enemy will have plenty of weapon to deal with heavy armor by simply ignoring it like Plasma Rifles, so don't be fooled by the Centurion and how it has an 2+ T5 save - If the enemy brings S 7 AP 2, there really isn't any reason to take them over another Tactical Squad. Also do note that the AP mechanic means gaining one point of armor is not simply 1/6 more chance of stopping your enemy's wounds : Not only will it do that, but against weapons that negated exactly your old armor save, you'll get a full new one. Also notice that, quite logically, best AP weapon get rarer and rarer. This is why armor gets exponentially better.

However: while armor saves are more like to get negated it's more likely for a soldier in 40k to snag a cover save (since they're not forming into rank and file to fight), which works just like an armor save in practice only reliant on terrain rather than something intrinsic to the model. So it's possible for even units with an armor save of 6+ to get a 4+ cover save. Of course these have their own counters most often, but often there are different sorts of counters, you rarely get low AP ignore cover weapons and when they do show up the cheddar rage is real.

That said, here are the armor saves:

• 7+: Can't even save against a stiff breeze/literally no protection at all. Found on only the likes of these shambling mooks and other such mass-deployed models. The only way to improve this is cover. Unless you're about to charge or don't mind a few deaths, don't let your models get caught in the open.
• 6+: A few plates here and there to stop the odd shot from killing the wearer, the relatively tough hide of a xenos beast, or leather braces to stop tiny flying rocks from slitting your wrists. Used mostly by the Orks and the Termagaunts and Hormagaunts of the Tyranids, and could just as well not be there—most weapons ignore them anyways. Other units like Kroot have this paltry save not to protect them, but as a final fail safe if they somehow end up outside of a forest. Everybody will laugh at it, especially weapons that will by definition be effective against their wearer—typically, low value / high number units—like flamers and grenades. Don't count on it.
• 5+: What would be flak armor today—can stop a normal bullet pretty reliably, but this is 40K we're talkin'. Mainly possessed by the Imperial Guard and their fuckloads of infantry, as well as Eldar Guardians. Better than 6+, but doesn't do much to save your sorry ass if something wants you dead. Treat like 6+: a better save but still ignored or severely reduced by an unreal amount of stuff (Pulse Rifles, Bolt Rifles, Gauss Flayers, etc). Still gives you a ~33% save chance against lasguns, shootas, frag grenades/missiles and multi-lasers, so marginally better than 6+ in fact.
• 4+: Futuristic armour covering most of the wearers body. In other words, Carapace Armor. Usually not mechanical or technologically advanced, however. The 4+ is a save you can trust more reliably: though still a toss-up, it is harder to ignore and a lot of sorta cheap troops have it, namely Fire Warriors, Necron Warriors, Tempestus Scions, and Space Marine Scouts, among others. It's also where a lot of light vehicles fall. Protects from bolters and their non-heavy cousins, which is a great plus. Probably the most cost-efficient armour around in both fluff and crunch, as 5+ is ignored by pretty much anything, and 3+ costs too much money/points and still suffers from the abundance of low-AP weapons. There are still things that ignore them, so watch out—you should be safe against most other troops, however.
• 3+: Power Armor that covers the entirety of the body with technologically advanced plate of awesome, as well as most tanks. This would be the sweetest save there were, as most Marines have it and others Elites tend to have it too, like the Crisis Suits and Immortals, if there weren't so fucking many "Marine Killer" weapons around. It certainly has the best usefulness ratio as it will protect against most if not all non-vehicle-mounted or non-specialized weaponry, while the next armor (2+) will probably not fare any better against those weapons that do ignore 3+. Will defend your dudes reliably, but keep away from Plasma and anything that ends with "-cannon".
• 2+: The granddaddy of Armor Saves, this is heavy Power Armor like Terminator Armor and the armor of super heavy vehicles (OR ONE GROT WIT DA BIG BANNA). Again, Fuckin' A if there are no AP-3/4 weapons around, but you'd be lucky to encounter that. Perhaps in acknowledgement of this, many units with a 2+ will also have some sort of Invulnerable Save to back it up, so that those expensive Terminators you fielded will still get at least some sort of save against the comparatively dirt-cheap Weapon Specialist squad throwing Plasma at them. Combined with the usually low number of troops carrying these kinds of saves, make sure you know how to keep the big guns from them—if you can you manage that, then you are golden.

Age of Sigmar/Warhammer_40,000_8th_edition

The latest version of both games have turned to a simple, hybrid version of both save systems. Today:

• All units have a given base save value.
• All weapons have a given modifier to save (Rend in AoS, AP in 40k), independent of who or what is using it/it is being used against.

And that's basically it. Roll dice, apply modifier, compare with base value: equal or bigger, wound gets negated.

An individual unit's special rules sometimes modify this a bit (e.g. Skeleton Warrior units have a +1 modifier to save if the enemy's weapon has a rend of 0) in order to differentiate units and armies, but those rules are kept simple in order not to bog gameplay down.