In computer security, a demilitarized zone (DMZ) or perimeter network is a network area (a subnetwork) that sits between an internal network and an external network. The point of a DMZ is that connections from the internal and the external network to the DMZ are permitted, whereas connections from the DMZ are only permitted to the external network -- hosts in the DMZ may not connect to the internal network. This allows the DMZ's hosts to provide services to the external network while protecting the internal network in case intruders compromise a host in the DMZ. For someone on the external network who wants to illegally connect to the internal network, the DMZ is a dead end.
The Security DMZ is used for providing external controlled access to services used by external personnel to the control system network control system equipment to ensure secure application of system updates and upgrades.