This page contains information regarding Star Trek: Strange New Worlds, and thus may contain spoilers.
The Mempa sector was a Beta Quadrant sector containing the Mempa system. The sector was under Klingon control as early as 2256. (TNG: "Redemption"; DIS: "Context Is for Kings"; SNW: "Strange New Worlds", "Spock Amok")
Several forces allied with Captain Kurn were stationed in the Mempa sector before the outbreak of the Klingon Civil War. When Worf convinced Kurn to support Gowron, Kurn had to go to the Mempa sector to meet with the other squadron commanders in order to convince them to support Gowron as well. (TNG: "Redemption")
Following the outbreak of war, Gowron's forces were able to destroy the Duras' forces' supply lines in the Mempa sector. Three weeks later, the forces loyal to the Duras family had rebuilt their lines in the sector, thanks to Romulan support, and defeated Gowron's forces in the Mempa system, forcing them to retreat to Beta Lankal. (TNG: "Redemption II")