Auxiliary Tanking

Auxiliary tanking is a system that reduces the damage an NPC or player deals to whoever it is attacking based on the amount and tanking quality of players and pets you have in front of it that are attacking it (with autoattack). In raid situations in particular this becomes of critical importance, making otherwise impossible DPS manageable.

The most any single character/pet can lower a mob's damage via aux is 15%. Both max HP and AC count toward auxing. The higher level the target mob is, the more HP and/or AC a character will need to aux at maximum effectiveness; the HP and AC of a fairly high-end (tier 9+?) tank will be needed for them to hit 15% aux effectiveness against a level 68 mob. (In terms of augs, an AC aug will add more toward auxing compared to an HP aug; each point of AC is worth 5 times as much as a point of HP.)

The most a mob can be auxed down is 20% of their base damage. However, higher level mobs start with a bonus; anything over level 60 gets a 20% bonus to their base damage at the beginning of the aux calc, and so will hit for 120% of their base damage if no one is auxing them. In other words, they'd need to be lowered by 40% to be fully auxed down. So you'd need at least 2 tanks and a melee at tier 9ish, or a tank and more melees. On red con mobs, at least. So if counting the bonus damage, a mob's total melee effectiveness can be lowered by 33% (120-40 = 80) at most.

Every class, pet and NPC can contribute to auxiliary tanking, but there is a hardcap on how low any NPC can go in damage. A check is made all defensive abilities (mitigation, AC, etc) with the exclusion of styles and factors that into how much aux percentage each character adds. For these reasons, since pure casters and priest classes don't have the best tanking stats around, they are usually better off standing out melee range where they are safe from Whirlwind, Riposte and other melee range damage.