I think the priority-order has varied over time, but you can easily find it out for the version of Selenium IDE that you're using. It's explicitly set in the last line of locatorBuilders.js, which currently reads:
LocatorBuilders.order = ['ui', 'id', 'link', 'name', 'css', 'dom:name', 'xpath:link', 'xpath:img', 'xpath:attributes', 'xpath:idRelative', 'xpath:href', 'dom:index', 'xpath:position'];
The reason it's done this way is explained by the comment immediately above that line:
// You can change the priority of builders by setting LocatorBuilders.order.
Ross