I think the name is a little confused.
The (sub)domain is actually the domain for our account. All our file is under the /(sub)domain/public_html/
The tech support staff in the "chat help" called it subdomain. It is a subdomain of x10host.com. But from the managing point of our free web account, it's a "domain" associate to our account. If I install wordpress with multisite option checked, then I may have subdomain under this (sub)domain.
As the (sub)domain is associated with the free account, I could not blank the domain nameand create a new one myself.
Sean0u