The Shimano Dura-Ace FH-9000 11-Speed Low Spacer is part of Shimano’s spacer lineup designed to ensure compatibility between various Shimano cassettes and freehub bodies. It’s intended for use when fitting 9 or 10-speed cassettes onto an 11-speed freehub body. For a 9-speed cassette, you only need the 1.85mm spacer (FW8991). If you’re installing a 10-speed cassette, you’ll require both the 1.85mm (FW8991) and the 1.0mm (FW4671) spacers to achieve the proper fit.