Safe zones are cities, towns or other communities under the formal protection of the Baltic Union government. They are usually well-fortified and large enough to field at least a battalion-strength force of men trained to Tier Two (i.e., professional military) standards.
The safe zones have their origins in the immediate post-Great War period, when the Baltic States militaries took control of areas with functional fallout shelters or comparable facilities of vital importance. These areas were designated "safe zones" under military protection, refugees with valuable skills being accepted and provided shelter and food in them. Granted, this protection wasn't extended to just any civilian out of pragmatic concerns, the very finite resources being reserved exclusively for those who possessed valuable skills that would be necessary to rebuild in the post-war world.
After the war, during the Colonel era, safe zones were redefined as areas capable of providing enough valuable resources and/or manpower to warrant full military protection.
Presently, the "safe zone" status is extended to any settlement capable of providing a battalion-strength force, or within the immediate area of operation of a battalion produced by such a settlement. Lesser settlements that acknowledge the authority of the Baltic Union government, but aren't awarded a safe-zone status, are still granted military protection, but are not eligible to delegate their candidates to government elections on their own, instead having to seek patronage of the nearest safe-zone government.