frontend_content_updated
frontend_content_updated is a hook which is called when a Frontend's content is updated. frontend_content_updated is similar to the hook frontend_content_saved, just is called only for updates.
This is hook very useful if you're using the cache feature. It will let you know when the cache should be deleted. Also, it could be used in the themes to change some options. When the hook frontend_content_updated is called, all registered hooks will receive the parameter content_id. You could use that parameter to delete cache or change the options.