Where is WindowsSDK_IncludePath defined?

The macro $(WindowsSDK_IncludePath) has the values shown in the picture.

I'd like to know where those values are defined, they must be defined in some files.

The picture was taken from Visual Studio 2013.

I see the data in file sdk.props in folder C:\Program Files (x86)\Windows Kits\8.0\build\CommonConfiguration\Neutral

<PropertyGroup>
     <WindowsSdkDir Condition="'$(WindowsSdkDir)' == ''">$([MSBUILD]::GetDirectoryNameOfFileAbove('$(MSBUILDTHISFILEDIRECTORY)', 'sdkmanifest.xml'))</WindowsSdkDir>
  </PropertyGroup>

  <PropertyGroup>    <WindowsSDK_IncludePath>$(WindowsSdkDir)Include\um;$(WindowsSdkDir)Include\shared;$(WindowsSdkDir)Include\winrt;</WindowsSDK_IncludePath>
  </PropertyGroup>

I use a Win8 + VS2012, so it should be in folder 8.1 for your VS2013 + 8.1 SDK.