AFAIK it's used for some AI weights (perhaps on CBs?), and maybe for some undocumented hardcoded interactions, but overall it doesn't seem to have been widely-used by the devs in the visible text files. If you're writing your own events, you could include weightings for rationality/zeal/etc in the way the AI chooses event options.What do we know about rationality, zeal etc? I found a bit on the modifiers page of the wiki but that's very basic information.