Defense to an attack makes it less likely to hit its target(s) but does not reduce its potency. If the power does not hit, then none of its effects apply. In most games, this is referred to as evasion.
"Every attack power is classified based on how the attack is delivered, and based on the type of damage it delivers, and every defense power has an associated type or types that represents what types of attacks that defense power is effective against." –Arcanaville
There are ten possible tags an attack can have. These tags fall into two conceptual categories: positional and delivery.
Positional (a.k.a. Vector) Tags
Three tags reflect the range and targeting nature of the attack:
- Melee Attack (includes both single-target and area attacks)
- Ranged Attack (single-target only)
- AoE Attack (also includes placed patches, non-melee cones, and non-melee PBAoEs)
Delivery (a.k.a. Damage) Tags
The other seven tags reflect the primary method used to deliver the attack's effects:
- Smashing Attack (for powers that work through blunt physical impact)
- Lethal Attack (for powers that involve sharp objects or high-speed force)
- Energy Attack (for powers that involve radiation, electricity, or some other untamed energy)
- Negative Energy Attack (for necromantic powers and powers that weaken life)
- Fire Attack (for powers whose effects are caused by flames or high heat)
- Cold Attack (for powers whose effects are derived from extreme cold)
- Psionic Attack (for powers involving mental manipulation)
Although the names are similar, delivery tags and damage types are different things with different meanings. Damage types come directly from a power's effects, and each instance of damage has exactly one type and is applied separately against its appropriate Resistance. Delivery tags are conceptual and were added by hand. Attacks can have multiple tags, and all the tags apply to the attack as a whole, which either entirely hits or entirely misses.
Although there are attacks that primarily cause toxic damage, there is no Toxic Attack delivery tag. There is no current way to specify that an attack works by using poisons or caustic chemicals. Attacks that would have this tag simply lack it.
Generally, an attack will have exactly one positional tag and either one or two delivery tags.
One fact to highlight about positional tags is that attacks are not classified as melee vs. ranged separately from single-target vs. area. A short-range attack that can hit multiple targets will be tagged as either a Melee Attack or an AoE Attack, but not both. Which tag it gets depends less on its range and damage and more on game balance and whether it looks like a melee attack.
- Example: Blazing Aura is an 8' radius PBAoE. Fire Sword Circle is a 10' radius PBAoE. Blazing Aura is tagged as an AoE Attack, while Fire Sword Circle is tagged as a Melee Attack.
Attacks that cause damage of all or mostly one type tend to get the matching delivery tag. An attack that causes two types of damage may get tagged for both or only one. A power that cause no damage may get no delivery tag, or it may get one appropriate to the power's concept or secondary effects.
- Every Energy Blast attack deals a mix of smashing and energy damage, and has both the Smashing Attack and Energy Attack delivery tags.
- Every Fire Blast attack carries only the Fire Attack delivery tag, even though some deal only fire damage and some cause a mix of fire and smashing.
- The Dark Blast set has the Smashing Attack tag only on attacks that do smashing damage, but it has the Negative Energy Attack tag on all its attacks, including one that causes purely smashing damage and one that causes no damage at all.
Remember, tags are arbitrary. The developers can give an attack any tags they want, regardless of its effects and other properties. They have added – and removed – tags from powers for conceptual and balance reasons. It's not possible to intuit correctly every attack's tags merely by looking at what it does.
Defense Tags and Stacking
Defense bonuses carry tags as well, to specify what styles of attack they work against. Each Defense bonus applies only versus attacks whose tags match one or more of its own. There is one additional type of Defense bonus called Base Defense, or DEF(Defense), which applies against all attacks regardless of tagging.
Every time an attack is made against a target, the game checks the target's total Defense bonus against each of the attack's tags separately. Once the game has those separate totals, it uses only the best one in its calculations that determine whether the attack hits. The important result of this rule is that, for any given attack, Defense buffs only stack when they all have one of that attack's tags in common.
- Example: Deflection Shield gives a Defense bonus against Melee Attacks, Smashing Attacks, and Lethal Attacks. Glacial Armor gives a Defense bonus against Energy Attacks and Negative Energy Attacks. A Tanker with both these buffs is ambushed by a nasty Council Vampyri who tries to hit her with Siphon Life. That power is tagged as a Negative Energy Attack and a Melee Attack. Our Tanker will only be protected by one of her two Defense buffs. Lucky for her, it will be whichever is larger.
+DEF(All) and Other Little White Lies
Some powers are described as providing Defense against all attacks. It's usually summarized as +DEF(All), or just +DEF. There are many subtly different ways that powers like this might be created. All of them behave differently in various rare circumstances, and almost none of them actually protect against every single attack in the game.
- The power provides Defense against all tags. This is the most common implementation. A defensive power built this way applies against all attacks except completely untagged ones, such as those made by the Hamidon.
- The power provides Defense against all positional tags. These powers give no protection against attacks with no tags or with only delivery tags, like Mind Control attacks.
- The power provides Defense against all delivery tags. A power built this way offers no mitigation against untagged attacks or against attacks with only a positional tag. This includes most primarily Toxic damage attacks, like Hydra spit and Zombie vomit, and many non-damaging control powers like Web Grenade and Seeds of Confusion.
- The power provides a Base Defense bonus. This kind of Defense applies to all attacks, period. Very few powers have the pure "defense" defense type, but Personal Force Field is an example.
+DEF(all but X)
Some powers are labeled +DEF(all but Psionic). These powers are like +DEF(All) powers in category #3 (and occasionally #1), only without the Psionic Attack tag. Those few powers that fit category #1 actually will provide protection against psionic attacks that have a positional tag.
Some powers are described by players as +DEF(all but Toxic). This is a misnomer, caused by confusion over the fact that the Toxic Attack tag doesn't exist, or sometimes used for clarity and simplicity while discussing effects with other players. These powers are actually +DEF(All) powers from categories #1 or #3. In particular, a +DEF(all but Toxic) power that fits category #1 will protect against attacks that do mostly Toxic damage if those attacks have positional tags (and most of them do).