Damar stands beside a map of the Federation-Cardassian border.
The Federation-Cardassian border (often referred to simply as the Cardassian border) was the division between the United Federation of Planets and the Cardassian Union in the Alpha Quadrant. For several years in the 2370s, part of the border was included in the Demilitarized Zone. (TNG: "Journey's End", "Preemptive Strike"; DS9: "The Maquis, Part I", "The Maquis, Part II"; VOY: "Alliances")
Cardassian star systems on their side of the border included the Unefra system and Chin'toka systems. (DS9: "Improbable Cause", "Tears of the Prophets")
Two additional systems, the Torg'Q system and the Ventani system were referenced in a deleted scene written for "Tears of the Prophets". The Torg'Q system was the closest potential target along the border to Cardassia Prime; however, it was also the most heavily fortified. The Ventani system was said to be far from Cardassia Prime and hold little strategic value.
Prophet's Landing was the closest Bajoran colony to the border. (DS9: "Heart of Stone")
In 2370, Commanders Benjamin Sisko and Calvin Hudson were the two highest ranking Starfleet officers stationed near it. (DS9: "The Maquis, Part I")
Later that year, the USS Prokofiev and USS Enterprise-D were dispatched to the Cardassian border after the illegal capture of Miles O'Brien by the Cardassians. (DS9: "Tribunal") Shortly thereafter, the USS Odyssey was on a patrol of the border. (DS9: "The Jem'Hadar")
Several Federation starships disappeared near the border after the Cardassians joined the Dominion in 2373, including the USS Tian An Men. (DS9: "In the Cards")
In late 2373, General Martok took the IKS Rotarran on patrol near the Cardassian border, to provide advanced warning of a Dominion fleet heading for Deep Space 9. (DS9: "Call to Arms")
In early 2374, a captured Jem'Hadar attack ship commanded by Benjamin Sisko encountered and was attacked by the USS Centaur on the Federation side of the border. Sisko hoped that by crossing it, Captain Reynolds would not pursue them, but the Centaur crossed the border and continued its attack. (DS9: "A Time to Stand")
Later that year, after Operation Return, the USS Cortez was destroyed near the border. Captain Sisko commented, that patrolling the Cardassian border was becoming more and more dangerous, since a starship could always run into a squadron of Jem'Hadar fighters. (DS9: "Far Beyond the Stars")
This border'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".