1 (asteroid belt)
The Border planets are near enough to the central planets that they have business dealings with those on the Core. However, the Border planets are far enough away from the Core that the eyes of the Alliance can't always make out what's going on.
Thus, these planets are excellent locales for certain unscrupulous folk from the Core to conduct business dealings "in private." They don't have to move to these planets (Though there are those eccentrics who travel here from the Core to 'get away from it all' - the kind of people who build strong fortresses to keep out the riffraff, and would never dream of socializing with the local yokels). The irony is that these same folk are all in favor of the rules and regulations that govern business dealings throughout the systems - just as long as those rules and regulations don't affect them.
To give the folk on these planets credit where credits are due, there are plenty on the Border worlds who are eager to do business with those on the Core. And there are always countless numbers without a silver in their pocket here looking for work.