The Windows 10 preview program had two settings: Fast and Slow. A third one has been added, called Release Preview, although it sounds a bit different from the other two. According to the blog post, which is supposed to be about a new build of Windows 10 Mobile, Release Preview will grant early access to updates on the current branch of Windows 10. They also call it “updates” instead of “builds”. Fast and Slow, as they have existed, provide builds for the next branch of Windows 10 at the time.
When the public was on July release, Insider provided builds that ended up in the November update. When Windows 10 1511 was released, Insiders received builds on the “Redstone” branch, to be released at some point in the future. That is, apparently, not how Release Preview ring will work. They will receive 1511 updates early. They might receive the final Redstone-one build before general availability, but that is just speculation.
This might take some pressure off of Slow, which, during the Threshold-two timeframe, only received a single build, 10565, outside of the final 10586 one that was released to the public. Slow ended up being little more than a release candidate ring for the upcoming branch. If they push that to Release Preview, for the build that the public will see, then Slow might receive a few more steps on the upcoming branch, especially now that Fast are receiving more frequent updates. After all, users who are only interested in one or two builds per branch will probably be satisfied with pre-release updates and the final build early (again, if they release the final builds early on Release Preview, which is speculation).
Or Microsoft might just want a few more testers for Windows Update patches. Who knows?