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
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.
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.