What Defines What Category A Post Picks (if in multiple)

If I have a post in multiple categories, what defines what category shows up in the URL? Assuming I have permalinks setup as /%category%/%postname% is it the ID or the name?

WordPress always use the category with lowest ID. You can use plugins such as the Hikari Category Permalink plugin to change that however.

http://wordpress.org/extend/plugins/hikari-category-permalink/