A tactical intelligence analysis from Outpost Sierra 6
Sierra 6 was a Federation outpost near the Romulan Neutral Zone. In 2366, this outpost detected the presence of a Romulan scout ship, piloted by Alidar Jarok, in the Neutral Zone. (TNG: "The Defector")
According to Star Trek: Star Charts (pg 66), Sierra VI was one of a series of outposts codenamed Sierra along the Neutral Zone. These outposts were located in the Kaleb (Sierra) sector in the Beta Quadrant.