Originally Posted by HTupolev

8/9/10-speed Shimano freehub bodies are all the same; 7-speed are narrower and 11-speed are wider.

A 9-speed road cassette will likely seat fine on an 8/9/10-speed hub without any spacers. If you put a 9-speed cassette on an 11-speed road hub, it will probably require a 1.85mm spacer inboard of the big cog.