From Guild Wars 2 Wiki
— In-game description
- Damage is dealt before boon removal if the Mug trait is enabled, so Venoms also trigger before boon removal.
- Does not steal "stacks" of the boons; e.g. the enemy has 15 stacks of Might, your character is only going to receive 1 stack of might (unknown which individual might application/duration takes priority); if the enemy is using a boon that stacks duration, such as Protection, and has both a 3s and a 4s application of it (showing as 7 seconds total), your character will only steal the 3s or the 4s application of the boon (unknown which application/duration takes priority), not the full 7s.
- Prioritizes aegis over other boons, followed by stability, protection, regeneration, retaliation, fury, swiftness, might, resistance, quickness and vigor.
- ^ https://forum-en.guildwars2.com/forum/professions/thief/Priority-Stealing-Resistance/first#post5174817 Priority Stealing Resistance?