What is this ring for that came with the MP3?

[BikeSlob]

I've ridden my new contraption for some miles now but not sure where it goes.
This might be related but my gear side (6 gear cassette from GM) of the tire is closer to the frame than on the disc brake side. The wheel turns true and balanced. Should I be evening this out with this big washer between the cassette and motor? Or with small oval-hole washers?

[Andrew]

It is a spacer that can go on the pie freewheel thread if you need to alter the position of your free wheel cog set.   Often not needed. Sometimes needed for certain design freewheel cog sets.
Andrew