In the language of MMO games, two or more effects or instances of an effect are said to stack if they can exist on the same target simultaneously. Generally, any two effects can be assumed to stack unless explicitly stated otherwise. If effects do not stack, it is probably because the effects are of the same type.
When more than once instance of a single effect is stacked, they are usually grouped together and a limit is set for how many instances are allowed at one time (an exception to this is when different sources cause the same effect, in which case each source will have its own group and limit). Since most effects are temporary, figuring out the duration of such a group can be tricky, but basically only three methods are used:
- Each instance maintains its own timer.
- When a new instance is added, the entire group adopts the duration of the newest instance. In other words, the new instance "refreshes" the group or "resets the timer".
- When a new instance is added, it adopts the duration of the group. (this method is very uncommon since instances added toward the end of a group's duration are virtually pointless)
Alternatives to StackingEdit
When more than one effect or instance does not stack, one of the effects must be favored. Sometimes the newer effect will replace the older effect, which usually happens when the newer effect is more powerful. Yet, in other cases the newest effect is simply ignored. (If the newest instance "resets the timer" of an effect without stacking, then this is the same as replacing it.)
In rare cases, multiple effects may work together to improve each other or even create an entirely new effect, which is a concept known as synergy. It is debatable whether or not this counts as stacking, since the new or improved effect(s) may or may not absorb the original effects, but that is an argument of semantics.