From EQOA News

General Rules:
There are 4 categories that a buff can fit into-and only four. No less, no more.

Category 1: Target
Most buffs were previously in this category. We'll generally refer to this as the "target" category, because most buffs that were left in this slot are single target abilities without a radius (so Area of Effect (AoE) abilities do NOT fit here).

Category 2: Self
There were a few buffs in this category previously, and generally, those were the ones you'd remember "stacking with everything" like pureheart. This category now is used for all self buffs. Items may or may not apply but going forward it's more likely than not they will be, pet or familliar buffs are generally in this category. If it's an ability that you can only cast on yourself, or an ability casted by the creation of an ability you cast yourself, it will probably be in this category.

Category 3: Group/AoE
There were a few buffs in this category previously, and generally, those were the ones you'd remember "stacking with everything" like lycan/werehunter buffs. This category is now home to all group and area of effect type abilities.

Category 4: Form Buffs
This was mostly in place already, but anything with an illusion of sorts attached to it is in this category.

How Stacking Works:

First, let's clarify that these rules only apply when we're looking at the same stat. We're not saying you can only have 4 buffs total, just 4 that buff the same stat.

Examples:

1. I'm a necromancer, and I'm using a self Armor Class (AC) buff (Ebonweave). I can't use any other self AC buff to stack with it (say, Maliceweave or Hateweave), but if a paladin joined my party and used their group AC buff, it would stack.

2. I'm an enchanter and I've cast my (Power over Time) PoT buff on the group. A bard in my party also casts an AoE ability on the group. The best ability will stay and the other one will not affect the group. However, the self PoT buff I have on me and the targeted PoT buff I've cast on my group will still provide full effects.

Damage Shields/Defensive Procs and Offensive Procs:

Damage Shields are the same as defensive procs. You can only have 1 of each active at one time. The stacking rules have not changed damage shield effects.

Debuffs and Damage Over Time:

Detrimental abilities and damage over time stack by archetype. For example, a shadowknight, a shaman, an enchanter, and a druid could all have damage over time abilities working at the same time now, instead of just the largest one.

Abilities of the Same Line:

Any ability that is an upgrade to an existing ability or simply changes the scope of the ability will NOT stack. This is the rule that generally trumps all other rules (see the exceptions section for more information).

Exceptions to the rule:

Some abilities do not stack in accordance with the target, self, group, and form buffs either due to the need to meet the "Abilities of the Same Line" rule, or because their effect would be more beneficial in a different category.

1. Cleric and Druid Armor Class/HitPoint (AC/HP) buff lines.
Clerics and druids both can purchase Class Mastery (CM) upgrades to change their AC and HP buff lines into group and AoE buffs (United Faith and Nature's/Vitae's Blessing). The cleric buffs have all been categorized in the "group" or third category, even the single target ones. The Druid buffs have all been categorized in the "target" or first category, even the group/AoE ones. This is to make sure they don't stack with buffs from the same line, but do stack with the other class's buffs to provide the maximum benefit.

2. Alchemist and Shaman Stat buff lines.
Alchemists and Shaman both can purchase CM upgrades to change their stat buff lines into group and AoE buffs (Concentration/Inspiration and Tribal Accuity/Unity/Spirit). The alchemist buffs have all been categorized in the "group" or third category, even the single target ones. The shaman buffs have all been categorized in the "target" or first category, even the group/AoE ones. This is to make sure they don't stack with buffs from the same line, but do stack with the other class's buffs to provide the maximum benefit.

3. Enchanter PoT tick upgrades.
Enchanters are the only current exception to the "Abilities of the Same Line" rule. By purchasing the Empathy CM line, they can use that buff in their group ability slot and still use the single target ability that was upgraded.

4. Form of the Master and Wolf's Instinct.
These two level 51 120 cost CM abilities stack in the "form" or 4th buff category, but no longer have an illusion associated with them.

5. Werehunter buffs.
Werehunter buffs were categorized in the slot where they seemed to have the best effects. For tanks and melees, they are category 4 (form). For casters and priests, they're category 2 (self).

6. Short tank AC buffs.
Not to be confused with gnome warriors and dwarf paladins, this is referring to some short duration buffs (like the 12-24 second AC buffs included in the various taunt paths). These are generally effective in the first or target category to provide maximum stacking with self and group buffs many tanks possess.

7. Necromancer Damage over Time (DoT) abilities.
Some higher level necromancer DoT abilities stack with each other, meaning they will NOT stack with other DoTs (as we still only have the 4 categories to work with).

8. Lycan buffs/Racial CMs and MCs/some MCAs (Master Class Abilities)
At the time of writing, some abilities have yet to be altered. Yes, Pureheart is one of them, as it’s an Immortal MCA. Please note that if it doesn’t stack properly, it will be altered in a future update.