NI was a registry number prefix for Federation starships operated by Section 31 in the 23rd century.
The four-digit registry of an unnamed starship began with the NI prefix, with the registry NI-1101. (DIS: "Through the Valley of Shadows")
Additional NI registries were mentioned in Eaglemoss Collections' collection of starship models. From the Star Trek: Discovery The Official Starships Collection, there was the Shiva class (β) NI-0047 (β), the Nimrod-class (β) NI-0064 (β), and the Hou-Yi-class (β) NI-1009 (β). An additional registry, from the Star Trek Universe: The Official Starships Collection, was the Deimos-class (β) NI-1021 (β).