International Zones is an addon to the security system in Airport CEO. It introduces an new type of zone that can be accessed via International checkpoints from a secure zone. This adds another layer of complexity to your airports.
International areas allow you to process international flights. The amount of international flights you get is determined by what country you place your airport in. I you place it in, for example, the US, you won't get a lot as there are lots of domestic airports in the US but, if you place it in, for example, Egypt, where there are few domestic airports, you will get more international flights. Note: when you chose a county in the schengen zone, domestic airports count as all airports in the schengen zone. This can be disabled in the gameplay panel.