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")
In the battle near Xahea, Control was finally dismantled and neutralized, and the USS Discovery traveled 930 years into the future to prevent it from reasserting itself. Starfleet subsequently destroyed everything that remained of it following the Discovery's' departure. (DIS: "Such Sweet Sorrow", "Such Sweet Sorrow, Part 2")
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 that 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") Although the Discovery destroyed Section 31 headquarters, Control survived. The crew of the Discovery believed that Control was infected by the same unknown AI from the 28th century that had infected Airiam. (DIS: "The Red Angel")
During the events on Essof IV, Control possessed Leland and attempted to kill Georgiou and transfer the Sphere data to its ship in orbit. However, the Discovery destroyed the Project Daedalus facility before more than 54% of the data could be transferred. Control was able to beam aboard the NCIA-93 and escape, however. (DIS: "Perpetual Infinity")
It then took control of the NI-1101 and killed its crew. Control then re-animated and possessed Lieutenant Kamran Gant. It posed as Gant and was at first able to trick Burnham and her brother Lieutenant Spock into boarding the ship by blocking their tricorders. However, Spock and Burnham discovered Control's deception. Control attempted to inject Burnham with its nanobots in order board Discovery and take the sphere data. Burnham was able to shoot Gant's body but Control attempted to use its surviving nanobots to take control of Burnham. However, Spock was able to magnetize the floor and destroy the remaining nanobots before they reached Burnham. (DIS: "Through the Valley of Shadows")
With the entire Section 31 fleet under its command, it attacked both the Discovery and the USS Enterprise. Before that, Control had disabled Starfleet's subspace relays, leaving the Discovery with ship-to-ship communications only. He then later beamed onboard the Discovery's bridge and searched for Sphere data while the Discovery prepared to travel into the future in order to keep the Sphere data out of Control's hands. As Control's fleet overwhelmed the two Federation ships, L'Rell and Siranna unexpectedly arrived with Klingon and Kelpien reinforcements. It came to a fight ending with Control being neutralized by Georgiou who threw Control into the reaction cube and then magnetized it, pulling the nanites from Leland's body. Control's ships were left dead in the water as a result and the Enterprise quickly destroyed them while the Discovery traveled into the future in order to prevent Control from ever reassembling itself.
Alternate realities and timelines
In an alternate timeline experienced by Gabrielle Burnham after getting sent to the 32nd century, Control absorbed the Sphere data and became fully evolved. As a result, it went on to destroy all sentient life in the galaxy. This timeline was negated by Gabrielle's intervention as the Red Angel and the departure of the USS Discovery to the future with the Sphere data after Control was neutralized. (DIS: "If Memory Serves", "Perpetual Infinity", "Such Sweet Sorrow", "Such Sweet Sorrow, Part 2")
Representations of Control have been portrayed by Tara Nicodemo, Sonequa Martin-Green, Anson Mount, Doug Jones, Alan van Sprang, and Ali Momen. Due to Control's possession of Leland, its primary representation was van Sprang.
In an interview with StarTrek.com published on 1 April 2019, actor Alan van Sprang echoed fan speculation regarding a potential connection between Control and the origins of the Borg: "I think it's very intriguing. When I first read the script I thought, 'Oh, is this the making of the Borg? Is that how it happens?' We're as much in the dark as anybody else, but as soon as I saw that, I thought, 'This is like The Borg.' The Next Generation's Borg episode just blew my mind [when I watched it originally], let alone when Picard became Locutus. That's the first thing I thought of, which kind of tickled me to no end. 'Wow, I'm just going to milk this for all it’s worth.'" 
In an interview with TrekCore.com published on 19 April 2019, Michelle Paradise, then writer and co-executive producer of Discovery, clarified: "It's interesting — 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." 
In the Star Trek: Section 31 novels predating Discovery's season 2 Disavowed and Control, "Control" 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). This Control was stopped by Julian Bashir.