Factions were born out of necessity, in a time of war, panic and chaos.Trust was a hard currency at that point. Five decades since, and those wounds have closed , but the scars remain. Factions are still mostly made up of the Tribes that formed them, and are not very open to outsiders. Yet, at about a 1 to 10 ratio in the main cities and 1 to 20 or less in the further ones, you will come across people from a Tribe that would normally belong to another Faction.It is usually curiosity, love or just self,preservation that brought them there. Although these immigrants are never treated aggressively (thanks to the Clover Treaty), they are almost always considered second,rate citizens. To what extent, that depends on the Faction's hospitality and way of life.