The Cuellar system was an inhabited Cardassian star system located in Sector 21505, in the Alpha Quadrant. It had a good strategic position, as it was "within arm's reach" of three Federation sectors alongside the Federation-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")
This star system was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the primary of the Cuellar system was a primary was a F-class star.