AR-558 was a planet in the Chin'toka planetary system. This system was located in the Alpha Quadrant. The terrain of the planet consisted primarily of rocky surfaces and caverns, and was generally inhospitable to humanoid species for any extended period of time.
During the Dominion War, the planet was the location of a Dominion communication array. Following the invasion of Cardassia (See: First Battle of Chin'toka), the planet was held by Starfleet troops for five months without replacements, as Starfleet hoped to use the captured array to tap into Dominion communications.
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 beseiged almost from the start. Repeated Dominion attacks sapped both force strength and morale. Even though the location of their strongpoint 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 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 by repeated Jem'Hadar attacks and the prevelance 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.
Presumably when the Chin'toka system was retaken, AR-558 was recaptured by the Dominion and abandoned by Starfleet.
The name AR-558 was an in-joke, as the episode's production number was 558.
According to the Star Trek: Star Charts, on the star chart 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.