The long strange trip
The Travellers’ Aid Society travel zone classification for a nation, world, or system which is dangerous to travellers. In general, the imposition of a red zone classification indicates the location is quarantined, interdicted by higher authority, or at war.
Quarantine indicates that a dangerous disease is present, and the danger of war is self-explanatory, but interdiction requires further discussion. Interstellar governments often find it necessary to restrict access to worlds or systems for political or military reasons, and do so by publishing interdictions. Enforcement of interdiction varies with the reason for the restriction. Interdiction may be imposed on a world if it is a military base or other sensitive installation, for the private reservations belonging to powerful families desiring seclusion, for developing societies which the government has elected to allow to evolve in isolation, or for valuable resource areas being saved for later development or exploitation.