A polity claims a concentric ring around Focus. The ring is normally centered on a planetary orbit, but may differ in the case of non-planetary polities (the Spacer Confederacy and Partisan Territory). The polity has exclusive economic rights within this border, and is the only entity allowed to set up permanent or semi-permanent structures (orbital colonies, unmanned research stations, etc).
However, areas outside a body's Hill sphere are considered neutral with respect to travel. As long as you don't enter a body's gravity well and don't plan to loiter, you can pass through without having to go through any red tape.
This is how Welkinstead is able to ferry troops and supplies between itself and its base at Moonlitter without having to set anything up with the Spacer Confederacy.