The Add-on domains feature lets you host several websites from a single hosting account, they will all be controlled through the same control panel.
The difference between Add-on domains and Parked domains is that Add-on domains don't have to be pointed at the root of the domain, they act as completely seperate domains, nobody will ever know there are mulitple ddomains hosted on the account.
Add-on domains must be registered domain names that you own and the nameservers must be set to match that of the main domain on your account.
To add-on a domain to your existing control panel:
1. Login to your Control Panel
2. Look for Domains and click on it.
3. On the right side of the screen there should be a "Create a New domain" Click on it.
4. Type in the domain name and make sure to uncheck or disable share document so that your new domain will have its own root directory.
5. Click Submit.