The hosted domains feature of a website hosting account indicates the amount of registered domains which you can add in the same account. Registering a domain address and hosting it are two completely different services although a lot of people consider them to be the same thing. While the registration ensures that you become the owner of a certain domain address, the hosting aspect is what in fact permits you to have a web site because this is where your data and email messages will be. Because these are 2 different services, you can register a new domain address with one company and host it with another one by changing its name servers (DNS) - the domain is going to work in the exact same way as if it was registered and hosted with the same company. Also, it is very important to know that changing the hosting means pointing the domain name to another company and not transferring it.
