City of Heroes/City of Villains is much more liberal than most MMORPGs are when it comes to stacking.
- Buffs and debuffs caused by different powers always stack, even if they all come from the same player or enemy.
- Buffs and debuffs coming from different players or enemies always stack, even if they all use the same power, with one exception. The buffs from any version of Vengeance do not apply to any characters currently buffed by any version of Vengeance. (Any player can get Vengeance from the Leadership Power Pool. Arachnos Widows can also get Tactical Training: Vengeance from both the Widow Teamwork and Fortunata Teamwork power sets. Nemesis Lieutenants have their own, special version of Vengeance that has no stacking limits.)
- Buffs from the same player using the same power multiple times usually do not stack. Instead, the second application replaces the first, with a brief but critical gap between erasing the old buff and starting the new one. One common exception where buffs do stack is protection buffs versus status effects, since the gap that would occur if they didn't stack could cause the buffed character to succumb to mezzing effects.
- There is no common rule for debuffs from the same player using the same power multiple times. Some stack, others don't. It depends entirely on the power.
Stealth is often spoken of as being stackable or non-stackable. That's not technically true. Stealth bonuses follow the same rules as buffs in general. Any Stealth bonuses you can actually receive will stack fully with one another.
What the game does limit is how many different stealth-generating powers a single player can use simultaneously. Generally speaking, all toggles that grant stealth to either you alone or to you plus nearby allies are mutually exclusive. You may only have one running at a time. Also included in this exclusive set are any Temporary Powers that completely change your appearance, even if they don't give a stealth bonus.
The mutually exclusive list does not include:
- intangibility powers such as Phase Shift
- Super Speed
- Stalker Hide
- any click powers
- any power that only grants stealth because it has a +Stealth IO Enhancement in it
You can use, or get stealth buffs from, any number of those powers in addition to one of your own exclusive stealth toggles. You also fully benefit from other players' AoE stealth toggles on top of any of your own stealth.