Originally Posted by
botto
sounds like the spacer, which should be put on before the cassette.
Yep, it's the spacer that goes behind the 10speed cassette on a 9 speed carriage. The 10 speed cassette is just slightly narrower than 9 speed cassettes so it needs s space to go on a 9 speed freehub body. You wouldn't need it if you're putting it on a 10-speed specific rear hub. Check which kind of hub you have first.