The Earth Outpost Stations were a series of at least eight outposts constructed on asteroids located in Sector Z-6, along the Federation side of the Romulan Neutral Zone. They were established by a Treaty after the Earth-Romulan War of the 2160s, which allowed United Earth, and subsequently the Federation, to monitor its side of the Neutral Zone.
In 2266, four of the eight outposts came under attack from a powerful plasma weapon used by a Romulan Bird-of-Prey; this sneak attack completely destroyed outposts 2, 3, 4, and 8. (TOS: "Balance of Terror")
Nearly a century later, a border outpost reported seven Romulan battle cruisers had come within its sensor range – a violation of the Treaty and even a potential prelude to an attack. The outpost sent out a distress call, to which the nearby USS Berlin responded. Starfleet also requested that the USS Enterprise-D join the Berlin to project a deterring show of force against the Romulans – which was also a necessary tactical move to ensure that, should a conflict erupt, the Romulans would not have the advantage. Nothing came of this, and the Romulans chose not to engage or even communicate with Starfleet until later that year. (TNG: "Angel One", "The Neutral Zone")
By 2367, the Neutral Zone outposts had expanded in number, possibly to as many as 23 (see Outpost 23). Additional outposts in the region included Outpost Sierra VI. (TNG: "The Defector", "Future Imperfect")