Apple today seeded the fifth beta of an upcoming macOS Sierra 10.12.6 update to developers, two weeks after seeding the fourth beta and over a month after releasing macOS Sierra 10.12.5, a minor bug fix update.
The fifth 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 features or notable bug fixes in the first four macOS Sierra betas, and because Apple does not provide beta release notes, we may not know just what's included in the update until its 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: A public beta is also available.