Figure I would finally take a picture of my most rare Mac CD's ever and share it here. I have not seen a picture of these discs anywhere on the internet and haven't seen any mention of them either so I figured I would share.
Its the release candidate for 10.0, its pretty much identical to 10.0 given to key developers to test out their software while the public was using the beta. Its pretty much a final release and falls between OS X DP4 and 10.0. I have never had a Mac that I can use it on so I don't know yet if it will update as if it was 10.0.
Where did you get it? I don't recall Apple ever using the "Release Candidate" label for system software. If you pop it into a OS X machine take a look at /System/Library/software_version. It'll show the version and build number.