The Ultimate Court of Appeal was the highest judiciary institution of the Time Lords of Gallifrey, its members being the supreme guardians of Gallifreyan law. (TV: The Ultimate Foe) It was appealed and involved during the trial of the Sixth Doctor, as plotted by the Valeyard. (TV: The Trial of a Time Lord)
The Sixth Doctor finds a hit list of all the members of the Court in the Matrix. (TV: The Ultimate Foe)
Xeroniam was one member of the Court. (PROSE: The Ultimate Foe)