SB-19 was an experimental project developed by scientists of Ni'Var in the 31st century, owing to the Federation's mandate that its member worlds research alternatives to dilithium. SB-19 would have allowed starships to be transported across thousands of light years in an instant, obviating the need for warp drive. As part of the project, sensors were scattered across dozens of light years of subspace.
Though Ni'Var scientists ultimately concluded that SB-19 was too dangerous, the Federation denied their request to shutter the program because it was the most promising dilithium alternative being pursued at the time. The Federation was subsequently decimated by the Burn, which Ni'Var believed was caused by SB-19. This led them to secede from the Federation and withhold the sensor data they had collected.
In 3189, the USS Discovery traveled to Ni'Var to request access to the SB-19 data, which would enable them to pinpoint the source of the Burn. President T'Rina initially denied their request, as SB-19 continued to be a topic of great cultural and political sensitivity on her planet. Commander Michael Burnham invoked T'Kal-in-ket in an effort to gain access to the SB-19 data by proving that the experiment did not cause the Burn, but withdrew her challenge after seeing that the proceedings jeopardized the unity between Vulcans and Romulans. Nevertheless, she convinced T'Rina that the Federation could be entrusted with the SB-19 data. (DIS: "Unification III")
Combined with the recovered black boxes of ships destroyed by the Burn, the SB-19 data allowed Discovery to trace the source of the Burn to the Verubin Nebula and they pick up a Federation distress signal coming from it. (DIS: "The Sanctuary")