Every civilization has borders and every civilization has members on the far ends of those borders. The Boundary Traverse is a society that is bound together only very loosely when at all. They are all of the ideal that civilizations grow because people push boundaries, and physical boundaries are included in this. There are Traversers in virtually every Civilization. They form a sort of border guard militia and early warning listening posts for their Civilizations.
"Frontiers" mean something different in galactic societies. Planets aren't being methodically colonized by most Civilizations - they jump from one system to one many light years away without settling the intermediary steps because certain planets seem more "useful" than others. The "frontier" of a Civilization might be all over the place, and Civilizations may well be threaded well into each other when looked at on a star chart. The "frontier" is anyplace no Civilization actively claims. These are the places that Traversers go to live.
They also form the first layer of trade relation between Civilizations, because while the Civilizations might have official trade negotiations, these generally account for a small percentage of the total product influx/outflow. The majority of it comes from the Traverser frontier settlers, who trade for useful supplies with each other. The Traverse was founded in part based on rules of negotiation and approach between settlers so that hostilities are kept minimal. As such, even Traversers who belong to Civilizations at war might negotiate with each other for trade, sometimes with the irony of the trade knowledges being used against the ones they got it from.
Benefit: Traversers gain +1 Twist, but only for Empathy Themes. Traversers also have access to "Conestoga" type space craft, capable of carrying a family and enough goods for that family to settle another world, making them good for small-quantity technical barter. They can also exceed one of their Capability limits by 1 without suffering the "sell-out" penalty to their Themes.
Core Values: The Boundary Traverse Core Value is Frontier Spirit. Traversers want to deal with other cultures and civilizations, rather than merely deal with only their own people. They want to find strange, possibly desolate places. They want to see the strange vistas of the world. More importantly, they want to settle those places and have their kids there. This might not keep them from shooting at anybody who approaches "their" time-space, but it will get them willing to talk before delivering a killing blow.