The shim referenced is not strictly a spacer, for adjusting cassette widths on
different speed hubs, it is way too thin. The only place that makes any sense
for it is as the first thing to put on the hub, then the cassette or not to
use it at all with a 10spd cassette. I had this conundrum on a recent
rebuild of a bent from 9->10 spd and found the shim under the 9 spd
cassette on the wheel. The 10 spd cassette initially did not like the shim
but a bit of fiddling made it work ok and I liked the resulting space
between the end of the lockring and the end of the axle so it was
left in place.