The Seldonis IV Convention, or simply the Seldonis Convention, was a treaty negotiated on Seldonis IV governing the treatment of prisoners of war. Under this treaty, torture was expressly forbidden. Also, a person held by a treaty signatory could request the involvement of a designated "neutral representative" in their case. The United Federation of Planets and the Cardassian Empire were signatories of this treaty, although the Cardassian Empire was known to violate it on several occasions.
In 2369, while undergoing torture at the hands of Gul Madred, Jean-Luc Picard reminded the Cardassian of this fact, but the gul ignored him. (TNG: "Chain Of Command, Part II")