OK, Shimano 10-speed cassettes come with a 1 mm spacer that goes behind the largest cog. It is required on all Shimano 8/9/10-speed freehub bodies (the few 10-speed only hubs don't require it). A Shimano 11-speed freehub body requires a 1.85 mm spacer to accept 8 or 9-speed cassettes and the additional 1 mm spacer to take 10-speed cassettes.