Sector 21503 is a sector in Federation space near the Cardassian border, and near sector 21505.
During the Cardassian War, while in this sector, the USS Stargazer made an unsuccessful attempt to offer a truce to the Cardassian Empire.
In 2367, while on a mapping survey mission, the USS Enterprise-D was attacked by a Cardassian Template:ShipClass ship in this sector because of earlier attacks by the USS Phoenix. (TNG: "The Wounded")