I am posting this here as information for any one like me who is having trouble figuring out what Co-Domain is and does. After a long search, what I found is that the name Co-Domain is probably not the best name for this feature. In English if you put co- before a word it means a separate but equal one. So Co-domain would mean an additional domain with the same features of the first domain. A better name in my opinion would be Aliases, like used in Apache. Alias means a different name for the same thing. So if you have a domain www.example.com then an alias would be example.com. I alos found this article which helps to explain What is Alias and What is Co-Domain. From http://www.smokinglinux.com/apache-tutorials/apache-alias-configuration-with-serveralias What is Alias? Alias is an alternative name to delivery a domain name. For example we have www.smokinglinux.com domain name configured correctly in our apache configuration files and we want other domains linking directly on our master domain or subdomains linking on www.smokinglinux.com. To know better this tutorial you have to see a configuration sample attached below: NameVirtualHost 127.0.0.1:80 <VirtualHost 127.0.0.1:80> ServerName www.smokinglinux.com ServerAlias smokinglinux.com web.smokinglinux.com DocumentRoot /var/www/smokinglinux.com </VirtualHost> How we can see in attached configuration code, we have a virtualhost code container with a ServerName and ServerAlias. ServerName is the main domain and ServerAlias is a set of alternatives names for main domain. In this case, when we try to reach this virtualhost by a browser, we can use “www.smokinglinux.com”, “smokinglinux.com” and “web.smokinglinux.com” to see the same website for every of these domain names. I hope the Moderators and developers do not take offence to this I am genuinely trying to help. I think that ISPConfig is fantastic.