Originally Posted by
FBinNY
The extra spacer is most likely a spare for use on freehub bodies which require a spacer behind the cassette.
There's also the unlikely possibility that the first sprocket's built in spacer isn't full width, so it might go there...
OP, how thick is the spacer?
If its a full 1mm, it is probably the former which FB mentioned as going on first, between the cassette and the inside freehub body flange in some but not all applications.
If it's thin, more like a shim than a spacer, the latter which FB pointed out, might need to be used between the smallest cog and the next one up the cassette.