The Seat is made up of components including this Seat Assy which is the most expensive.
Parts such as the Seat: Washer(8.5x24x2.3), Bolt.muf Sprt F, Bolt, Seat Assy are included in the Gsx600f Katana 2006 (k6) Usa (e03) Seat parts fiche.
In this schematic the "Cushion" is used most often.