Sector 12 was in Earth's galaxy but distant from Federation space. According to Tarrant, Federation survey ships sent to the sector tended never to be heard from again.
It was the location of Spaceworld and the home of the System. Astral point 781 was also in Sector 12 close to Spaceworld itself. (B: Redemption)
The black hole containing Crandor was also in Sector 12. (C: Dawn of the Gods)