The Cuellar system was a Cardassian star system located in Sector 21505 in the region between sectors 21166 and 22079. It had a good strategic position to be used for an invasion of three Federation sectors alongside the Cardassian border.
A Cardassian space station was located in the system until it was incinerated by the USS Phoenix in 2367. The Cardassians claimed it to be a science station, but Captain Benjamin Maxwell of the Phoenix claimed, that there was nothing of scientific interest in the system and considered it a military supply port. (TNG: "The Wounded", "The Mind's Eye")