This article is quite interesting, though it unfortunately is ended abruptly with some missing files around page 7. Besides that, its quite enlightening about the AI processes in Halo 2.

In order to create even more realism in how AI interacts with objects in the world, the team uses animation to smooth out the look of the pathfinding. “We also tag objects with animation points,” Chris explains. “A crate may have a tag that tells the AI, ‘You can jump on top of this if you want to.’ So when pathfinding the AI can decide to make a path over tagged objects. In combat, if the AI knows an enemy is hiding behind that tagged object, they may choose to jump on top of it and say something scary, or if they have lost sight of the enemy they may decide to jump up on an object to get a better look.”