The hosted domains feature of a hosting account indicates the amount of registered domains which you can add in the exact same account. Registering a domain address and hosting it are 2 different services even though many people consider them to be the same thing. While the registration ensures that you become the owner of a given domain, the hosting part is what actually enables you to have a website as this is where your files and e-mails will be. As these are 2 separate services, you are able to register a new domain address with one company and host it with another one by modifying its name servers (DNS) - the domain address is going to work in the exact same way just as if it was registered and hosted with the same company. It's also very important to know that changing the hosting means pointing the domain name to an alternative company and not transferring it.