Article 15 was a legal document used by a non-Federation member to make a request to join the Federation.
In 2381, Grand Nagus Rom decided to submit a formal application for the Ferengi Alliance to join the Federation. Admiral Vassery believed that Rom was desperate for Federation resources due to Nova Fleet disrupting Ferengi trade routes. During the negotiations, Rom and Leeta angled to make a massive profit as "signing bonuses," but Captain Carol Freeman added a provision of her own that the Ferengi must first recruit one other planet themselves for the Federation. After Rom gleefully signed, confident that he could easily fulfill that requirement, Freeman revealed the fine print that the specific planet was Qo'noS, the Klingon homeworld, quoting the eighth rule of acquisition. Impressed that he'd been swindled -- thus proving that there were those in the Federation who respected Ferengi culture -- Rom had Freeman bring him the standard forms to sign. However, this was only the first step to the Ferengi joining the Federation. (LD: "Parth Ferengi's Heart Place")
In 2401, the Singularity transmitted this article in full to the interstellar alliance after requesting help from Admiral Jean-Luc Picard. The Borg Queen was subsequently granted provisional membership in the Federation so that her faction could remain as "the Guardian at the Gate" of a massive transwarp conduit that threatened the Alpha Quadrant. (PIC: "The Star Gazer", "Farewell")