The hosted domains feature of a hosting account indicates the amount of registered domains that you can include within the very same account. Registering a domain and hosting it are two totally different services although a lot of people consider them to be the very same thing. While the registration ensures that you become the owner of a certain domain name, the hosting element is what actually allows you to have a site as this is where your files and emails are going to be. Because these are 2 independent services, it is possible to register a new domain name with one company and host it with another one by modifying its name servers (DNS) - the domain is going to work in the exact same way as if it was registered and hosted using the same company. It's also extremely important to know that changing the hosting means pointing the domain to a different company and not transferring it.