When a user visits a web page, the web server sends a response code in addition to the page content. A normal, non-error response has code 200. If the page does not exist on the site, the response code is 404. If the page exists, but the user is not authorized to visit the page, the response code is 403. The core software provides default responses for both 403 and 404 codes, but if you prefer, you can create your own pages for each.