From my understanding, each "site" appears as a different install (even though it may use the same DB/files) using a different sub-domain/domain. As such, your licensing requirements are the same as if they were separate installs on separate domains.
If you use any specific 1st party add-ons (Gallery, RM, etc) then they also would require to be licensed for each "site" that exists using that add-on.
As for appearances, even though there is one install, it appears to be multiple sites using that add-on
As an example:
Ford.myforum.com
Chevy.myforum.com
Dodge.myforum.com
All 3 use the same DB/script files via the add-on. They have an appearance of different sites since the content is different. They EACH require a license. Same with any add-ons that may be installed (like the RM or gallery or even most 3rd party add-ons).