You can create subdomains for any domain you own. For example: If you currently have 'example.com' registered with us, you can create subdomains such as: 'sub.example.com', 'sub2.example.com', etc.
In order to add a subdomain, follow these directions:
From the main ChangeIP website:
Note: You will not be charged for DNS Manager orders.
Now you can click on 'DNS Manager' in the Quicknav (on the right-side of the screen) and manage your subdomain from there.