In the Official Traveller Universe (or OTU) the Third Imperium rules the stars, but it does not rule the planets of the Third Imperium. Each planet is free to run things its own way and each planetary government has jurisdiction out to Low or Near Orbit. Where a planetary government has jurisdictional claims to other bodies within its local star system, all such jurisdictions are deemed interconnected via a legal fiction, with the Imperial Navy patrolling and protecting the balance of the star system.
Every main world has a designated starport. This may be a Downport on the planet's surface, or a Highport in orbit, or both. A planet may have other subsidiary landing or docking facilities but these are always defined as Space Ports. While Space Ports come under local control (either by local government or by private individuals or corporations) and local jurisdiction, all Starports come under the control of the Imperial Starport Authority and are considered Imperial Territory.
The Extrality Line, or XT Line, is the official boundary of Imperial Territory on any planet or surrounding a Highport in Near Orbit. If an individual can cross the XT line into Imperial Territory, then they are no longer subject to the laws of the host planet. Consequently, the XT line is usually heavily patrolled, on both sides, whether the starport is an A Class trade and shipbuilding hub, or an E Class ferrocrete slab with a tin shack control tower and a beacon array.