Subdomain is something that is part of your main domain. When you joined Stanford Domain, you selected a domain (ie. profx.su.domains). This is your main domain. If you want to create websites for econ101.profx.su.domains, econ201.profx.su.domains, etc., you can login to the cPanel to create these subdomains.

Note that you don’t have to create subdomains to have multiple websites. You can create a new website within your main domain such as profX.su.domains/econ101, or profX.su.domains/econ200. You don’t need to create subdomains.

AddOn Domains in cPanel is a feature that allows you to manage additional domain (non-stanford.edu, non-su.domains) such as mrsmith.com. You can use this feature if you have purchased a domain from a third party. In addition to adding an AddOn Domain, you need to work with your third-party vendor to update the DNS records pointing to Stanford Domains.