Sovereignty refers to a Corporation’s control over a Star Map. Each Station is located in a System, which is part of a larger Constellation and Region.
When a Corporation captures a Station, it takes control of the System. If all Systems within a Constellation are captured, the entire Constellation falls under the Corporation's control. Finally, once all Constellations in a Region are captured, the Corporation fully controls that Region.
This tiered structure of sovereignty provides Corporations with significant strategic influence, and the battle for control over these territories is a key aspect of gameplay.