The extraterritoriality line, the divisor between jurisdiction of a starport and that of the host world
The extraterritoriality principle dates back well past the First Imperium, and its lessons were not lost on Cleon I and his advisers.
In the Imperium, goods often must transit several star systems to reach their destination. If each star system is allowed to tax the transshipping goods, a huge barrier to trade results, because the final price becomes too high. So Imperial Starports create "free trade zones" - indeed, most observers agree this is their primary reason for existence.
But instead of stopping there, the Imperium took this right to suspend tariffs and duties and used it as the basis for suspending all local law, because the extrality principle has also proven extremely useful in matters not related to trade. The XT line makes the whole of the starport a safe haven for off-worlders. If nothing else, this provides a refuge when the natives turn hostile. Allowing Imperial companies and organizations a place to conduct their business free from local interference not only allows them greater influence on that particular planet, but also allows them to coordinate their activities across wider stretches of space. Everyone from mega-corps to humanitarian aid agencies may have their offices in the starport on hostile worlds.
Imperial authorities may also use the safety of the XT line to protect more covert activities. All manner of "cultural attaches" or other undercover intelligence agents may use the legitimate activities of the starport to conceal their own nefarious doings.