An add-on domain is a combination of the parked and subdomains, and allows you to host an additional domain within your account, but with separate content. Example Usage:

• You have two domains myhosteddomain.com and myhosteddomain.biz
• myhosteddomain.com is the main domain for your website and you want people to be able to access a *different* site as myhosteddomain.biz
• You upload your additional website into a directory in your webspace called /site2/
  you can now access the 2nd site as myhosteddomain.com/site2/
• You can add the additional domain as an "add-on" from within your Control Panel, redirecting it to the site2 folder.
• Now when someone types myhosteddomain.co.uk they get the original site and when they browse to myhosteddomain.biz they will go to the site2 website.