"Port Authority" is an administrative and gameplay concept that encapsulates which player/hierarchy of players has the right to control Movecraft access to a Town. Port Authority is controlled via the Outlaw system, where Town officials can designate criminals as Outlaws and prevent them from accessing their public docks and other Town amenities. By default, this ability is only available to a Mayor upon Town creation. Hostile Towns/Nations are also excluded.
Port Authority can be delegated to any Resident of a Town by its Mayor by granting Sheriff or Assistant rank. This permits that Resident to designate Outlaws. Towns that join or are conquered by Nations, however, share their Port Authority with their Nation. Nation officials can set Outlaws at the National level that denies access to Ports in all Towns of that Nation. A conquering nation, therefore, can enforce trade embargoes with hostile groups, including the original Nation of a given Town. This also applies to Neutral Towns, even when peacefully occupied.