What is an "Addon Domain" in cPanel interface?

With the Addon domain feature in Cpanel you can create a separate domain next to your main domain on your account. The HTML contents of a subdomain goes into a subfolder of the /public_html/ folder in your account.

Every addon domain also creates a subdomain. An Addon is basically a subdomain that can also be accessed by the full domain. So for the visitor it is transparent, because they only see the full domain in the URL bar.

So if you would add a domain named kat.com and your main domain is dog.com, Cpanel creates a subdomain called sub.dom.com. The contents which goes in the /public_html/sub/ directory will be visible via your browser under:sub.dom.com AND dom.com

You can create POP mail accounts with dom.com and also the FTP can be accessed with dom.com (user for FTP should be sub@dom.com)

Make sure that you also update the nameservers for your addon domain at your registrar with the same nameservers as for your main domain (see welcome e-mail).