The Seven Systems was an area of space in which Gallifrey and Levithia were located, (PROSE: The Twins in the Wood) as Gallifrey was known as the "Shining World of the Seven Systems". (TV: The Sound of Drums)
Black hole drives were banned in every corner of the Seven Systems. (COMIC: Pirates of Vourakis)
Peri Brown, as a Warrior Queen, ruled the planet Krontep and its Seven Systems, and was personally the Governor of seven worlds. (PROSE: Bad Therapy)