Base 755 was an Emerald Chain outpost in 3189. This was their closest base to the Federation Headquarters.
In 3189, after retreating from the Federation Headquarters, Minister Osyraa, the Emerald Chain leader, ordered a course to Base 755 for the commandeered USS Discovery and the Viridian, where they would link up with Chain reinforcements. However, Discovery was re-taken by her crew and the Viridian destroyed before they could reach the outpost. (DIS: "That Hope Is You, Part 2")