Can't find it in Windows SDK and DDK, but some of our existing code includes it. The only meaningful link I found by Google is: http://msdn.microsoft.com/en-us/library/windows/hardware/hh450839%28v=vs.85%29.aspx What's it?
Answer
This is a new header file, currently only present in the SDK/DDK of Windows 8 Consumer Preview / Visual Studio 11 Beta. I think this header is for splitting the Win32 API into Desktop/MetroApp 'partitions'.