sub-server or sub-domain

Using the GPL version, I wonder why there is a sub-domain option, and what distinguishes it from the sub-server option which can also create a subdomain.

The biggest difference I see is that I've so far been unable to create a sub-domain wiht email, unless I create it as a sub-server.

Perhaps the DNS handling is supposed to be different (but presently isn't), since a sub-domain could be delegated from the parent zone, or reside within the parent zone; whereas a sub-server would need to have its own zonefile somewhere.