Previewed at WWDC, launching in the fall.
Apple Seeds Sixth Beta of macOS Sierra 10.12.6 to Developers and Public Beta Testers
The sixth beta of macOS Sierra 10.12.6 can be downloaded through the Apple Developer Center or through the Software Update mechanism in the Mac App Store.
We didn't find any significant feature changes, design tweaks, or notable bug fixes in the first five macOS Sierra 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.