The Archanis sector was a region of space in the Beta Quadrant, near the Federation-Klingon border. The sector was originally claimed by the Klingon Empire, but was relinquished to the United Federation of Planets a hundred years before 2372. It contained the Archanis system, containing the planet Archanis IV, as well as a number of Federation starbases and military installations. (DS9: "Broken Link"; DIS: "Context Is for Kings")
In late 2372, Chancellor Gowron demanded that the Federation relinquish the sector to the Klingons, at the urging of whom he believed to be General Martok. (DS9: "Broken Link") In reality, this Martok was a Changeling who had replaced the real Martok over a year earlier. A Klingon task force took the sector shortly after, starting the Federation-Klingon War. (DS9: "Apocalypse Rising")
Before the episode "Contest Is for Kings", StarTrek.com placed the Archanis sector next to the Bajoran sector. The map "Alpha/Beta Quadrant Overview" seen in Captain Gabriel Lorca's ready room aboard the USS Discovery used the reference works Star Trek: Star Charts and Stellar Cartography: The Starfleet Reference Library as source materials. These works first placed this sector in the Beta Quadrant.  The map is responsible for establishing the sector as being in the Beta Quadrant, and in Klingon space at least at this point of the war.