Windows Insiders in the fast ring on PC gets their first Windows 10 Preview build 16170 from the Redstone 3 branch. Dona Sarkar, in her blog post, announced the availability of build 16170 for the Windows Insiders.
Though there are no new major changes in this build, but has some one core related changes and refactoring done for restructuring for the next branch work.
After finishing the current branch and making it for public availability, Microsoft has started development of the next feature branch named 'Redstone 3'. 16170 is the first build for the Windows Insiders in the first ring to get their hands busy with new changes.
As it's just the start, there are no such feature changes except the code restructuring, fixes and few changes & improvements. As it is from development branch, there exists few known issues. The change log shared by Dona Sarkar in her blog post mentions about few bug fixes in this build for Cortana and High DPI settings. It also has some changes for share icon in File Explorer and Night Light settings.
On the other hand, the known issues reports about 'Narrator' which does not work in this build. Double clicking on the Windows Defender icon does not work in this build. The game bar flashing bug reintroduced in this build.
Though there are no major issues in this build, still check all the list before you start installing it. In case, you are not ready to experience more bugs, you can as usual opt out from getting the initial builds and then come back once the branch gets it stable position. In case you didn't yet receive the build 16170, check out the Windows Insiders Program Settings to get the required configuration updates.