Sector 21528 was a region of space.
In the mid-24th century, the location of the Cardassian-Federation border running through this sector, and the adjacent sector, was in dispute. The Cardassians believed the placement of this border was the result of a calibration error by a reference beacon. According to them, the border should have been 4.7 light years closer to the galactic center.
After the Cardassian Central Command announced its intentions to annex the Minos Korva system in Sector 21527 and install orbital outposts around three planets in this system, the Federation responded, backed by the assignment of three Excelsior-class starships to this sector, with a strongly worded objection. The Cardassians did not follow through their intentions. (TNG-R: "Chain of Command, Part II")