Gross Subsector Product
Sector economies are difficult to track or regulate. With communications limited to the speed of travel, interstellar trade is very possible, but central control of such economies is highly problematic. Higher nobles effectively delegate local economies to dukes and other appointed leaders. The subsector economy can far more effectively be regulated and directed. For such reasons, the biggest effective interstellar stock exchanges run at the subsector level.
It cannot account for illegal or black markets. Illicit trade is not accounted for.
The Office of the Emperor, the Imperial Bureaucracy, the Imperial Interstellar Scout Service, the Travellers' Aid Society, the Merchant Service, the Educational Institutes, and many other interstellar organizations use this data for governance, interstellar economy, world surveys, and other necessary tasks.
Such data drives Charted Space.