Control was Section 31's threat assessment system. It relied on artificial intelligence and was located in a data center at Section 31 Headquarters. (DIS: "Project Daedalus", "If Memory Serves") After the Battle of the Binary Stars, Section 31 accelerated their threat assessment program to prevent a war from ever even starting. (DIS: "Through the Valley of Shadows")
After the war, the way the system was used was that Starfleet admirals fed data into Control, which then made recommendations. However, the final decisions always included the experience and instinct of the former. When a series of red burst signals began to appear, Admiral Patar started lobbying Starfleet to turn its decision-making entirely over to Control. (DIS: "Project Daedalus")
Philippa Georgiou felt that Starfleet relied too much on computerized threat-analysis, whereas in the mirror universe where she was from, the artificial intelligence took orders from her rather than the other way around. (DIS: "If Memory Serves")
Recommendations by Control
As Section 31 personnel prepared to use a memory extractor on Spock, it was noted that Control wouldn't proceed if there was any risk to Spock. However, Georgiou later claimed the machine would destroy Spock's mind. (DIS: "Light and Shadows")
In 2257, Control betrayed Starfleet and massacred the crew of Section 31 Headquarters by shutting off life support, though it was able to hide this fact by creating holograms of Admiral Patar and other Section 31 leaders and using them to communicate with Section 31 and Starfleet. (DIS: "Project Daedalus")
While Starfleet was searching for Lieutenant Spock and Commander Burnham later that year, what appeared to be Admiral Patar informed Leland and Georgiou that, since Spock required vital medical aid, Control's threat-analysis system would prioritize off-planet Vulcan medical facilities. (DIS: "If Memory Serves")
After a Discovery boarding party arrived at Section 31 headquarters and learned of the events that had transpired there, it was found that Lieutenant Commander Airiam had been corrupted and was attempting to give Control information about artificial intelligence gathered by the Sphere. To stop her, Commander Nhan was forced to eject Airiam into space. (DIS: "Project Daedalus")
In the Star Trek: Section 31 novels Disavowed and Control, "Control (β)" of Section 31 was an Artificial Intelligence which evolved within the computing systems of a covert United Earth (later Federation) surveillance network known as Uraei (β).
Michelle Paradise noted that, in regard to speculation that Control was connected to the Borg that "we weren’t thinking Borg at all. I mean, we talked about all sorts of different things in the room, but there was never any intent on our part to parallel that in any way. I can certainly understand why people started to think we were going in that direction, but it was never where we intended to go with it."