I was arguing with a colleague here than all references to domain and website should be changed to hostname (or more accurately fqdn) but understandably they highlighted how customers would still be asking where the domains are.
Last I checked domains and sub-domains and sub-sub-domains are all the same (I'm unsure how nameserver detection and DNS is handled for this). Given the previous threads about big changes to the way containers/websites and domains/aliases/hostnames are displayed/named/managed I decided to wait until getting too deep into explaining the problems and limitations.
Should sub-domains be treated differently to domains from a plan and billing and limit perspective? Not in my opinion, their resource costs are so minimal, it's the containers/websites/emails and other associated things with the resource cost.