I have three doubts. Anyone there for help?
It is an MSBuild property that controls the specific version of the platform toolset that you want to use.
More info here: http://msdn.microsoft.com/en-us/library/windows/hardware/hh454232%28v=vs.85%29.aspx
Most likely you want to use the last stable Windows SDK. You can check the toolset that you are using looking at your Visual Studio project file (.vcxproj), looking for this XML entry:
<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'"
Label="Configuration">
<ConfigurationType>Driver</ConfigurationType>
<DriverType>KMDF</DriverType>
<PlatformToolset>WindowsKernelModeDriver8.1</PlatformToolset>
</PropertyGroup>