can a user be configured with more than one "top-domain"(like ***.com) and can have some "sub-domain"(like ***.abc.com) under each "top-domain". besides this single user has a fixed disk quota
Hi Rona,
To rephrase you’re question – you’re asking if a single user can be an owner of multiple top-level Virtual Servers?
If that’s what you’re asking – a reseller in the Pro version can do that.
Beyond a reseller – no. As of today, a user can only manage one top-level Virtual Server (and all of it’s sub-servers).
You can of course take a top-level server, and move it underneath another parent Virtual Server, which would allow that user to be able to manage both.
-Eric
Actually, it sounds like Rona thinks names have meaning to Virtualmin. They don’t.
You can have a virtual server (named anything you want; say, "virtualmin.com), and that virtual server account can manage as many sub-servers (named anything you want; say, "webmin.com", "usermin.com", "software.virtualmin.com") as you want.
The notion of a "sub-domain" is meaningless to Virtualmin. Virtualmin does not care about names. A sub-server can be named anything you want, including a second level domain, like domain.tld. A virtual server also be named anything you want, including a sub-domain, like sub.domain.tld.
Don’t imagine that Virtualmin cares about names, and all of this will become immediately obvious. 
And to be clear: A reseller is not for this purpose. A reseller is for high level creation and management of multiple virtual server accounts (and those accounts can then manage virtual servers). It is for…resellers of hosting services.