Lockring spacer Shimano cassette
The "lockring spacer" (so labelled on Shimano Exploded View diagrams and other technical documents) is a very thin metal washer or shim between the lockrings of higher- end cassetes such as XT, Ultegra, and the like. Normally it is held captive on the lockring,  and based on Q and A from other forums (see example here: https://forum.cyclinguk.org/viewtopic.php?t=122213#p1235924) this little bugger is often installed unseen or the inexperienced-but-fully- prepped-with-technical-manuals mechanic is given lengthy pause as the search for this "spacer" commences. 
It is included with the appropriate cassettes in it's correct position.  As mentioned, higher end cassetes utilize this washer/spacer/shim between lockring and first position, high, sprocket as a means of protecting the integrity and functionality of the serrations of the associated lockrings, which in these upper-tier models is not steel but rather aluminum-alloy. This thin little washer then, "protects small peaks of those serrations getting worn off by cassette removal and replacement."   A more in-depth reasoning: "There are about 100 teeth in the serration and the screw thread is ~1mm pitch which means that each 'click' results in ~10 microns (less than half a thou') more compression. However the serrations are about 250 microns high which means that with metal spacers in the cassette, the lockring may never tighten enough unless the metal shim is fitted. Plastic spacers are more elastic which allows the lockring to tighten more easily. However with plastic spacers in the cassette they can 'relax' over time which results in a reduced clamping force and can often allow the lockring to go up about another 1/4 turn once the thing has been installed for a while (enough to cause problems with indexing accuracy in fact). Metal spacers don't relax but both metal and plastic spacers can wear thinner if the sprockets start to move around (which they will do, all the time, if the lockring doesn't provide an adequate clamping 
force)." 
So there's some light shed on the very slim shim. Look (closely) for it on your next top-end cassette.