Let's say that you register another domain. When you host that domain (by adding it as a domain in the dreamhost control panel), you can choose as an option to fully host that domain. This is probably exactly what you did when you hosted those subdomains (I'm guessing, but you tell me!).
When you do fully host that additional domain, if you choose the standard options, you'll see it as another subdirectory of your home directory just like your original domain and all those subdomains you created and hosted.
Does that help?
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