SOA and forwarders - Help for an exam please.

[David Skilton]

I'm studying for a Networking exam and in one of my text books it makes the
following statement :

The root name server of a domain is the domain name server that acts as the
Start of Authority for that zone. Moreover it is a server that forms the
top-level server in your domain. As such, it contains the "." domain and
thus can't be a forwarder. To resolve queries outside of your domain, you
should set up a forwarder, which will eventually have a cache full of
information. When a DNS server cannot resolve a query, it moves, (escalates)
it up to a root server that is authoritive for a zone. The start of
authority (SOA) record is the first record in the database.

I don't understand why a top level server in my domain "contososo.com" can't
be a forwarder even though it is the only DNS server in my domain. It's
probably a simple answer, but I just don't quite get it......