Defines the current mode of an AI. The most significant mode here is probably Dead (also used as ‘Unconscious’), which basically turns the AI off, and is checked by various scripts. Efficient and Super Efficient are used when the AI is far from the player (distances defined in AI→AI Core→Efficiency Settings (Property)). This turns off unnecessary things like animations, and at the furthest distances, stops the AI completely.
Thief handles this property itself ingame, so you’re probably best off leaving it alone.
Property Name: AI_Mode