Sector 9 was a top secret program of the Kal-Bryant Mining Conglomerate with the cooperation of the Terran Dominion. It was tied deeply into Project Shadowblade. The full nature of the program was kept secret, but advanced starship cloaking systems were developed from it. These systems used an advanced antiwave system, aided by the psionic powers of the ship's pilot, to cancel out all noise made by a ship's engines.
After Project Shadowblade shut down, the Dominion attempted to remove those with knowledge of the project, including those in Sector 9. Toom, a vice president who oversaw the project, was forced to commit suicide by the Terran Dominion after Aal Cistler sold secrets of the project to the Umojan Protectorate and framed Toom. During his time at the Ghost Academy, Tosh helped Toom uncover the truth about Sector 9, but Toom was mind wiped.
Due to his immunity to memory wipes Tosh retained knowledge of Sector 9. When he joined the spectre rebellion, intelligence and technology from Sector 9 helped was provided by Tosh to aid General Cole Bennett against the Dominion.
References
Kenyon, Nate. (September 27, 2011). StarCraft: Ghost: Spectres. Simon & Schuster (Pocket Star). ISBN 978-1439-10938-0.