Currently a license is purchased for a domain and this allows that domain and any subdomain for live and also that domain as a subdomain as another domain for staging purposes, ie a license for "mydomain.com" can be used on a staging domain like "mydomain.com.devdomain.com".
This works fine for http traffic, but if you want to test over https this requires an SSL certificate which it is possible to use a wildcard SSL, but most wildcard SSLs only support a single level of subdomain, which we currently use 2 levels. Need to look at any side effects of allowing a dev domain with the period removed so you could have something like "mydomaincom.devdomain.com" instead.