When you join Stanford Domains, you will be asked to pick a domain (ie., profX.su.domains). You will then to be able to have unlimited websites such as class101.profX.su.domains, class201.profX.su.domains by using SubDomain icon on your cPanel. If you have a need to have a second subdomain.su.domains, you would need to log a ServiceNow service ticket.
Note that you don’t always need to create a subdomain. You can install a second WordPress website in a sub-folder of your main domain. For example, your main domain is profX.su.domains and you are teaching two classes this quarter. You can have two WordPress websites called profX.su.domains/cs101 and profX.su.domains/cs201.