On your cPanel you can find the terms Domain, Subdomain and Addon domain. The primary domain assigned to the cPanel hosting is referred to as the main domain.
You can also assign additional domains to your cPanel, called addon domains. Each time an addon domain is added, a subdomain is automatically created for the main domain.
Example:
You order a cPanel subscription with testdomain.eu as the primary domain. You purchase the domain name testdomain2.eu and want to add it to your cPanel. In this case, testdomain2.eu is an addon domain, which when added will automatically create a subdomain: testdomain2.testdomain.eu. So sticking with the example in this case:
testdomain.eu - main domain
testdomain2.eu - addon domain
testdomain2.testdomain.eu - subdomain
