Guild Wars 2

For instance if a monster is condition capped by a player with high condi damage, will a low damage condi build replace his stacks?

Yes a low dmg one would replace those stacks. This is why condi dmg is horrible for PVE as well as a lot of professions get a minor trait where they bleed on crit, even if not spec ing into any condi dmg theyll overwrite your stacks

Intensity-stacking boons and conditions (might, bleed, vulnerability, confusion, torment), as you probably know, go up to 25 stacks. If someone applies yet another stack when the target is at full capacity already, the oldest stack is removed (even if it did not last its full duration) and the new, possibly weaker one, is applied.

For duration-stacking boons and conditions, this is where condition damage (and healing power when it comes to regeneration) comes into play. I will copy paste something I wrote a few months back:

There are also hidden timers when stacking the duration of boons and conditions (poison, regeneration, etc.). If I have higher condition damage than you and apply poison for 4 seconds on a target and you apply it for 3 seconds, the condition icon will show 7 seconds and the first 4 will be calculated with my condition damage and the rest with yours. If I apply poison in the meantime, it will continue to tick with my damage and “your” 3 seconds will be on standby until I stop adding poison to the mix. In other words, the condition duration is divided between sources and it lets the person with the highest condition damage tick first until his source runs out. If he comes back and applies a condition later on, he gets a free VIP ticket to condition heaven and starts dealing the damage again.

You can see the current source of the condition by hovering your cursor over the icon. If no source is listed, it means you’re the one ticking right now.