Effects stack in one of two ways: in intensity or in duration. For example, multiple stacks of bleeding, which stacks in intensity, increases the damage dealt over time, and multiple stacks of regeneration, which stacks in duration, increase the duration that the healing over time lasts. All effects can have a maximum of 25 stacks, whether intensity- or duration-stacked, with exceptions noted below.
The game tracks each application of any effect separately in terms of duration and intensity. Those standard values get improved by the applying character's attributes (Condition Damage and Condition Duration for conditions, Boon Duration for boons).
Removing a condition or boon will remove all stacks at once.
 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.
- 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.
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.
 List of duration-stacking effects
The effect from the source with the greatest intensity will be used for calculations for the duration of the effect. If there are two unique sources of Burning, the effect with the greater condition damage value will be used until that source ends, at which point, the intensity from the second source is used for the remaining duration. Similarly, swiftness does not stack intensity with other effects that increase movement speed.
Boons are limited to a maximum of 9 stacks, with the exceptions of Retaliation and Stability. The intensity (duration) of each stack is irrelevant.
- Retaliation (max 5 stacks)
- Stability (max 3 stacks)
- 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.
- 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 tick; 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.
- Damaging effects (retribution, bleeding, poison, burning, 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.