Effect stacking refers to the how an effect increments upon application. Players encounter effect stacking often due to boons and conditions, which both have effects that stack. Effects that have charges, such as conjure weapon effects, aren't generally considered stacking if the effect only decrements. Effects that stack duration are more numerous than those that stack intensity.
To explain this it is helpful to clarify the term "stack". In addition to the general meaning, in an MMO context the word "stack" is used to refer to each object in a stack (each application of the effect is "a stack"). So you may have "ten stacks of Might", which is the same as saying that the size of your Might stack is ten. A clearer way to express what is actually happening is to say that you have ten applications of Might.
With that in mind, effects in Guild Wars 2 may stack in duration or intensity. One can think of duration as stacking horizontally and intensity as stacking vertically. A duration stack cannot be made "higher", but it can be made "longer". An intensity stack can be made "higher", and while it may grow longer (because intensity-stacking effects have duration) the stack itself only grows vertically. Each application of an intensity-stacking effect applies and expires independently of and simultaneously with other applications. Applications of duration-stacking effects apply sequentially and only one application is effective at a given time (see examples below for further clarification).
- Examples of duration-stacking effects
- 10 seconds of regeneration are applied to a target from a level 80 player with 1500 Healing Power. 5 seconds later, 10 seconds of regeneration are applied to the same target from a level 80 player with 2000 Healing Power. The target will experience 317.5 health per second for the first 5 seconds, then 380 health per second for the next 10 seconds, then 317.5 health per second for the next 5 seconds. [Does this mean higher-intensity versions have priority and "interrupt" lower-intensity versions? If so, the paragraph above is wrong when it says they "apply sequentially". Does this happen with all duration-stacking effects, such as burning, or is it specific to regeneration? Clarification would be useful.]
- A Guardian stacks swiftness by repeatedly casting Symbol of Swiftness. Once 9 stacks of swiftness have been applied, no further sources of swiftness can be given until at least one stack has expired. Symbol of Swiftness applies 4 seconds of swiftness per stack; 9x4 = 36 seconds of swiftness. Until one of those stacks is gone, at 32 seconds, other sources of swiftness, such as Save Yourselves will not apply any additional duration.
- Example of an intensity-stacking effect
- 10 seconds of bleeding are applied to a target. 1 second later, another 6 seconds of bleeding are applied to the same target by a second source. For 6 seconds, the target will take damage over time from both stacks of bleeding. After that, the second bleed effect will expire, and for 3 seconds the target will continue to suffer the bleeding from the first effect, which then expires.
For duration effects, if the same effect is present from multiple sources, the source with the greatest intensity will be used up first. When that effect expires the effect from the next most powerful source will resume and use the amount of time it had remaining when the more powerful effect started (which can be its entire time if it started while the other source was on top of the stack). Most effects have a stacking limit, which means that the effect can only last so long, and further stacks cannot be applied while the stack is at the limit. This limit applies to each source independently, as multiple sources can stack the effect for longer than a single source.
Effects that stack in intensity can stack up to 1500 times, except might and vulnerability which are limited to a stack size of 25. The sources with the greatest intensity and number of stacks will take priority in calculating damage. Removing effects, usually with condition and boon removal, ignores the intensity of the effect.
Appearance in the user interface
The duration of an effect is indicated by a border around the effect icon that gradually disappears as the duration decreases (the icon itself vanishes when the effect ceases). The number of applications of the effect is indicated by a number in the bottom right of the effect icon. The duration shown for an intensity-stacking effect is from the source with the greatest duration.
List of duration-stacking effects
Swiftness does not stack intensity with other effects that increase movement speed. Boons are limited to a maximum of 9 stacks, with the exception of Retaliation.
List of intensity-stacking effects
Each stack applies its own intensity for its listed duration, independently from the others, except if the condition is removed as a whole.
- Bleeding (max 1500 stacks)
- Burning (max 1500 stacks)
- Confusion (max 1500 stacks)
- Poison (max 1500 stacks)
- Vulnerability (max 25 stacks)
- Stacking sigil effects
- The effects gained from some sigils which increase a character's attribute with every kill, such as the Sigil of Bloodlust, stack in intensity.
- Damaging effects (retribution, bleeding, poison, burning, torment, and confusion) of a specific source will update in intensity if the source's stats change by various means (e.g might).
- However, same does not apply to Regeneration. Using Banner of Tactics will not update the next healing ticks from regeneration if banner was applied after the regeneration was.
- Currently no information on how stacks are dropped when extras occur.
- Absolutely no rounding happens for duration totals (tooltips only round for display) but it is unknown what happens at certain split seconds for tick based effects.
- Some skills like Well of Corruption can transform boons into conditions or vice versa.