A Demilitarized Zone (D-Zone) is where a player cannot take any damage from any Alien or enemy player if the player did not start the attack. Common places for a D-Zone are a company base, or certain portals. You will know exactly when you are in a D-Zone if you see the text "Demilitarized zone" appear. Once on a D-Zone, you do not get the neutrality advantage. To get it back, you must either destroy the enemy that is attacking you or jump the current portal you are at.
When in a Demilitarized-Zone, you must not attack or be attacked for a short time before being protected.
Having the D-Zone does not apply to all portals located throughout the maps. Any Battle Map has free open portals that do not have D-Zones, and enable enemy players to attack freely. Being on an X-4 map is the only method that gives you the D-Zone, but once you are to jump the portal into a Battle Map, it is gone. [Which is an advantage to whoever "runs" the map. Which means that you gather our company, outfit, or clan and wait in a giant group near the portal, and then you attack whoever enters. Creating a devastating force.