Domain hosting terminology

I have registered and found a host (pair.com) for five domain names.

Fortunately all five are for the same customer as they goe through 
the process of decide what web address they want.

It surprises me that all 5 domains reflect the same html pages. 
Something I wanted but didn't know how to do. I thought I'd have to 
have 5 folders, each with the domain name and separate index.html 
pages.  Is it because they are all in the same account?

But now I want to delete all but two domains and have them reflect 
each other, and then add a new and third domain that is totally 
separate from the others.  A new account for the third domain?

What terminology do I use to help me learn what it is I have and what 
it is I want.

Also, can a Domain mirror only a specific topic of an already 
existing domain (with approvals)?

Pair.com is willing to help, I'm sure, but I don't know what words to 
use.  Perhaps there is reading material you can refer me to?

Linda

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