Party AI

Aggressiveness: willingness of party to attack. I imagine TW put this in because if you recall in early versions of M&B parties would chase after every wandering farmer, so general offensives would get nowhere.

Courage: willingness of party to defend.

Initiative: readiness of party to do anything. I made some general rules:

Party attacking enemy: initiative 100.

Party default: initiative 80.

Party just following marshal or occupying/patrolling own center: initiative 50.

Party dug in at siege: initiative 20.

Marshal gathering army: initiative 10 (Native)

This last is why I appoint a screening party even when marshal is just gathering.

EDIT: The low siege number is why besieging parties do not run off when defensive relief arrives.

As far as I can tell from the MS, it determines how likely will parties pursue opportunity targets instead of continuing with their current AI state. The number may be a formula calculated from the distance, strength ratio and speed difference with the enemy party. At least that makes most sense.