Sector 18 was a specific area within the Trymant system. The Trymant system itself was a star system located in both the Outer Rim Territories and the Western Reaches regions of the galaxy. During the time known as the High Republic Era, the Galactic Republic used a satellite, specifically Republic Security Zone Monitoring Satellite 777B-Alpha, to keep watch over sector 18.
Back in 232 BBY, the Great Hyperspace Disaster triggered what were called Emergences. These Emergences were the sudden returns from hyperspace to realspace of pieces from the destroyed Legacy Run freighter, which then crashed into planets and other objects all over the galaxy. At that time, a Dug worker stationed on the monitoring satellite was at their workstation, talking to themselves and observing that nothing of interest was happening in sector 17, sector 18, or sector 19. However, the Dug's attention was quickly diverted when they spotted debris from hyperspace entering the Trymant system nearby. They then informed their captain, who ordered an immediate evacuation and sent a warning to the Starlight Beacon space station.
The first mention of Sector 18 occurred in the first issue of the Star Wars: The High Republic Adventures comic series from 2021. This comic was part of the larger Star Wars: The High Republic project's initial Phase I. Daniel José Older wrote the issue, and IDW Publishing released it on February 3, 2021. Prior to the release, on November 17, 2020, StarWars.com published an article to announce the comic series, which included a preview that contained the reference to the sector.