The Zerg Quarantine Zone is a region of space of the Koprulu Sector, presumably designed to keep the zerg at bay from the rest of the sector. Appropriately, Char is located within this area.
Elder, Josh and Ramanda Kamarga. "Why We Fight." In StarCraft: Frontline: Volume 1, pp. 6–47. Tokyopop, August 1, 2008. ISBN 1427-80721-3.