Previewed at WWDC, launching in the fall.
Apple Seeds Third Beta of macOS Sierra 10.12.6 to Developers [Update: Public Beta Available]
macOS Sierra 10.12.6 can be downloaded through the Apple Developer Center or through the Software Update mechanism in the Mac App Store.
No significant features or notable bug fixes were found in the first two macOS Sierra 10.12.6 betas, and because Apple does not provide beta release notes, we may not know what's included in the update until it sees a public release.
macOS Sierra 10.12.6 is likely to be one of the final updates to the Sierra operating system as Apple transitions to macOS High Sierra, which was introduced at the Worldwide Developers Conference in June.
Update: The third beta of macOS Sierra 10.12.6 is also available for public beta testers.