Outpost Delta was a military observation station on the border between the territories of Gallifrey and the Monan Host. It was near a gravitation rift that distorted light and time.
The outpost was staffed with members of the Temporal Powers, but, at the start of Livia's presidency, it was occupied by only Time Lords and Warpsmiths due to increasing hostilities with the Monans. While coordinator of the Celestial Intervention Agency, Romana II sent Ace to investigate whether the Monans were preparing for war.
There, General Gaal was Ace's liaison, and after President Livia declared war on the Monan Host, a fleet of battle TARDISes led by Trave arrived at the outpost. After he planted a bomb in Ace's bedroom, with the intent to frame her for treason, Gaal told her that a Monan bomb had been found in an inaccessible area; he attempted to make his escape while she was distracted. In fact, there was no Monan bomb, but with warning from Leela, Ace was able to find Gaal's bomb in her bedroom and launch it out of the airlock. The bomb was pulled safely into the rift, but Ace was killed by the Watchmaker.
When Plutus tried to escape Gallifrey after being shot by Leela, Livia redirected his TARDIS to Outpost Delta, where Trave arrested him; the arrest order arrived there before him thanks to the rift. (AUDIO: Enemy Lines)