The Seldonis IV Convention was a treaty that governed the treatment of prisoners of war.
The Convention was negotiated in 2248 between the Federation and the Klingon Empire in order to deal with prisoners of war. In time, every major space-faring power in the region became a signatory to the treaty. (Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook)
After a covert operation into Cardassian territory which resulted in the capture of Starfleet Captain Jean-Luc Picard, Commander William Riker attempted to envoke the Seldonis IV Convention in order to prevent his interrogation by the Cardassian Union. However, Gul Madred countered this move by stating that the Convention governed only prisoners of war, which meant that the covert operation had been authorized by the Federation, which it denied. (TNG episode: "Chain of Command, Part II")