If you need to fit a XD cassette to a XD-R freehub body you will need a spacer on the body before you fit the XD cassette. A spacer is supplied with the body but if it is lost then any 10 speed spacer (as used to fit a 10 speed cassette on a Shimano HG 11 speed freehub body) will work just as well. One of these can be easily sourced from most decent LBS's.