These would over-ride the standard aggressiveness of the stack (if set for a particular player). Also normal diplomacy rules would be disregarded. So you could set a wandering stack to always join (for free, and what percentage), always join (for money, and what percentage), always run away (without chance to chase down, like banes in H2 Archibald campaign), always run away (with chance to chase down) or always attack (hatreds), and have different rules for different players. Even better if the map-maker could edit these variables during the game.
Note: this would be customized when editing a particular wandering monster stack in the editor, although global things like alliances with a particular creature type, banes (certain creature types always run away from a certain player with no chance to chase them down) or hatreds (a certain creature type always attacks a certain player, possibly without chance to let them go) could be options too.