Minecraft wiki entry for Building_a_metropolis, associated text: Containment
Useful if your country is in anarchy or high unrest. There are several "safe zones" in the city/country.
The safe zones are controlled by the police and/or army.
If you want, you can quarantine them. This means making it impossible or hard to get in or out.
The places which are not safe zones, are anarchy, (civil) war, apocalypse, etc.
Those are not controlled by the government and you can do whatever you want there.