Secondary Domains are “full-fledged” domains, that are not attached to the master domain in any way except that the feature limits are shared between the master domain and secondary domains. This means that if a hosting account has the ability to create 10 e-mail boxes, the master domain could have 5 e-mail boxes, and a secondary domain could also have 5 e-mail boxes, and that would be the limit for these accounts. Secondary domains have their own statistics pages, bandwidth recording, and web space.
How to: Add a Secondary Domain