AR-558 was a small barren planet in the Chin'toka system. This system was located in the Alpha Quadrant. According to "Tactical Map 4364W", this planet was located approximately twenty-five light years from Ferenginar, thirty-five light years from Starbase 375, forty light years from the Badlands, forty-five light years from Bajor, and fifty light years from Cardassia. The terrain of AR-558 consisted primarily of rocky surfaces and caverns, and was generally inhospitable to humanoid species for any extended period of time. (DS9: "The Siege of AR-558", "When It Rains...")
During the Dominion War, the Dominion established a communication relay station into a rock face on a steep mountain on this planet. It was the largest communications array in the sector. Following a victory in the Chin'toka system, (See: First Battle of Chin'toka) the Dominion was driven from the array by Starfleet troops who hoped to use the captured array to tap into Dominion communications. The array was held by these troops for five months without replacements.
The planet proved very difficult to defend. The Chin'toka system was the site of repeated confrontation, making re-supply to the planet impractical for extended periods. The Federation troops guarding the array selected a suitably defensible position at the closed end of a subterranean cavern, but were besieged almost from the start. Repeated Dominion attacks sapped both force strength and morale. Even though the location of their strong point forced the Dominion to advance through a narrow canyon, the casualties suffered by the Federation rendered it impractical to take full advantage of the terrain and forced them to wait for each successive assault while hoping for re-supply.
Although there were initially one hundred and fifty personnel posted there, by the time the USS Defiant made a supply run to the planet in mid-2375 that number had been reduced to only forty-three surviving troops from repeated Jem'Hadar attacks and the prevalence of subspace anti-personnel mines, nicknamed "Houdinis". It was only when the crew of the Defiant, working with the troops, were able to disarm the mines and re-purpose them to attack advancing Jem'Hadar that the Federation gained some semblance of advantage in the situation. The casualties were heavy, however, and included Captain Loomis, Commander Parker, Lieutenant Larkin, Crewman Kellin, McGreevey, and Vargas.
AR-558's orbital position in the Chin'toka system is unclear. A map of Chin'toka, seen in "Tears of the Prophets", depicts at least four planets, two of which are clearly the two binary planets contested in that episode. This planet may be either of the two remaining planets or the moons of any of the planets.
According to Star Trek: Star Charts ("United Federation of Planets I"), AR-558 was treated by the author, Geoffrey Mandel, as two separate locations. Both locations were in the Alpha Quadrant. The first was an antenna/communications relay. The second was a planet in a binary star system of two F-type stars. This planet was the site of a battle.
For Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), AR-558 was located close to the Chin'toka system in the Alpha Quadrant.
The Star Trek Encyclopedia (4th ed., vol. 1, p. 37) locates AR-558 in the Chin'toka system.