In Star Trek, Section 31 is a shadowy organization of the Federation that has its own infrastructure outside of Starfleet. Use with the [star-trek] tag and any relevant series tags.
First introduced in star-trek-ds9, Section 31 is a shadowy organization that operates outside of public eye of the federation. It has been depicted in star-trek-enterprise and star-trek-discovery, as well as the "Abrams timeline" (alternate).
Section 31 (named for Article 14, Section 31, which enabled its creation) is known for operating outside the rules and ethics that the Federation and Starfleet typically abide by. It has its own infrastructure, including a fleet of its own ships.