Must subdomains be registered just like domains? For example, if I have a Hostony account with 10 subdomains, must I register each subdomain I use for an additional $15 or so?

Thanks in advance for your response!

-- TH

Subdomains (subdomain.domain.com) just point to a folder on your site (domain.com) so no, they don't need to be registered.

It depends on account what you have.
For example you have an account with 10 subdomains.
If you want 5 additional subdomains you should pay 4$/mo
All informaition is on this page Hosting plans

Usually you need register subdomains. But sometimes you need to do it if you aren't owner of 2nd level domain name (for example domain.com). Some hosting companies provide their customer with subdomain for hosting (e.g. mydomain.hostony.com) and sometimes this service is not free. But if you are owner of 2nd level domain you need not to register the subdomain additionally. With you hostony you can have subdomains (limit of subdomains is according to your package) online without any fees.