Hi jm321.
The extra domains would be Add-On Domains.
The url for subdomains would be subdomain1.domain.com, subdomain2.domain.com, etc.
Before you add add-on domains to your web host, make sure you change the nameservers from the registrar's to your web hosting account. For example, if you bought a domain from godaddy.com, log-in to your godaddy account, modify the nameservers for the domain to that of your web hosting plan.
Let us know if you have questions about this
Sherif