This is kind of a development document trying to get some of my ideas about human interstellar settlement into order. It’s both kind of in-character and out-of-character as I haven’t quite worked out the specifics of the fictional technology and society involved.
In the early days, most interstellar settlement was in support of research facilities. Most research went into trying to study how Earth might respond to changes. This lead to a bimodal distribution of early settlement. The largest peak was lifebearing planets very much like Earth. The second peak was lifebearing planets very, very different from Earth. Continuing communication and ultimately return to Earth was a priority, so most of these settlements were fairly near to Earth.
There was a third, much smaller, peak in settlement in those early days. This was settlement by people who just wanted to get away from Earth for some reason. Some were drawn by romantic ideas of an untouched frontier. Others simply sought out a place to carry out their own utopian experiments(or build cults) in isolation from what they saw as the tyranny or corruption of society on Earth. Many were simply people with antisocial tendencies avoiding the dictates of authority above themselves. These people were often willing to brave long harrowing voyages in the hopes of maintaining their isolation. After long journeys to place themselves far from Solar civilization, these people could often afford to be picky about where they set down roots, choosing the nicest worlds. On the other hand, many of these people pushed their ships too far and had to settle on the first marginally habitable world to avoid being lost somewhere in space. This led to a somewhat flatter probability distribution with a peak at the most Earthlike worlds and tapering smoothly to increasingly unpleasant places.
As starships grew faster, permanent settlement became more common. Most people tried to settle closer to Earth in order to take advantage of the routinely accessible amenities of Earth and the core worlds.
There were still people who, for whatever reason, wanted to escape the influence of Earth and its civilization. Theses people continued to push their ever faster ships to try to avoid the spread of the greater civilization as long as possible. Many of these settlements were planted in the hopes of being able to establish dominance over a region of space before the expanding wave of civilization broke over their new worlds. Others, more realistic, simply sought to grow their population to the point where their world could remain independent of the greater civilization. There was always a residuum of settlers who believed, if they could just get far enough out, they might never again be under the dominance of Earth’s civilization.
Although the speed of interstellar travel continued to increase, the outer limits of common human settlement actually began to slow down. Early ships might have taken weeks to get to the closest stars and later ships could travel much longer distances in shorter times, but as conveniently reachable distances increased, the number of desirable worlds increased as the square of the speed increase. What this meant was that, if a handful of worlds was reachable for early ships, vessels ten times as fast could reach a thousand times as many worlds. While some of the problems of maintaining a cohesive civilization may scale with time lag of communication, other problem simply scaled with the number of entities involved. This became especially pronounced as interstellar society began to encounter earlier waves of separatist settlements. Some of those earlier settlements grew away from the sentiments that had earlier driven them away from the mainstream of human society and integrated fairly well, while others diverged still further in isolation.
Once the technology of interstellar travel became more mature, even the rate of increase in speed became less pronounced, stalling the wave of general expansion. Coupled with the still slow expansion from the various far-ranging outlier settlements, this led to the Ragged Edge, an area of spotty civilization at the edge of human settlement and exploration.
Hopefully, this was useful and not too confusing. I think it is probably a plausible pattern of settlement at the frontier of an interstellar civilization that has not yet approached any natural bounds.