The communication desired in the use case above is achieved
by configuring security policies that point to or from an
external zone.
An external zone is a security object that is associated with a
specific virtual system that it can reach; the zone is external
to the virtual system. A virtual system can have only one external
zone, regardless of how many security zones the virtual system has
within it. External zones are required to allow traffic between
zones in different virtual systems, without the traffic leaving
the firewall.