Sector 21505 was a region of space in the Alpha Quadrant. This Cardassian sector contained the Cuellar and Kelrabi systems.
In 2367, the rogue starship USS Phoenix assaulted several targets in this sector, including a research station in the Cuellar system. The USS Enterprise-D entered this region by order of Starfleet to assist the Cardassians in locating the Phoenix. The Phoenix successfully destroyed two Cardassian ships before it was stopped by the Enterprise-D. (TNG: "The Wounded")
This sector was only mentioned in dialogue.
This sector's quadrant of origin is inferred based on the position of the Cardassian Union as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends".