SDK releases are the life blood of Windows developers because it is how they gain early access to changes that are being implemented for the next major feature update to Windows 10.
Right now Redstone 4, the code name for that feature update which is being tested by Windows Insiders, should be out in March 2018 according to Microsoft's twice per year release schedule.
That means developers have about three months to implement these changes for their apps in preparation for that release.
The latest SDK build, 17040, was made available to developers last week and is compatible with any Windows Insider Redstone 4 client build above build 17040. That includes last week's release of Windows 10 Redstone 4 Build 17046 to Insiders in the Fast Ring.
According to Microsoft, any attempt to install this SDK release on any version of Windows 10 that is not an Insider build is not a supported scenario and could fail.
In this release the C++/WinRT headers and compiler are now part of the Windows SDK.
The compiler comes in handy if you need to consume a third-party WinRT component or if you need to author your own WinRT components with C++/WinRT. The easiest way to get working with it after installing the Windows Insider Preview SDK is to start the Visual Studio Developer Command Prompt and run the compiler in that environment. Authoring support is currently experimental and subject to change.
The ModernCPP blog has more information and a deep dive for the CppWinRT compiler plus Microsoft plans to provide additional how to's on using the compiler in the near future.
See the Windows 10 SDK Preview Build 17040 release notes for all of the API changes in this build.