What’s the difference between DirectX Tool Kit and legacy DirectX SDK ? UE4 uses which of them?
DirectX ToolKit is a library replacing part of the deprecated D3DX library set (Direct3D Extension). You can read more about here: http://blogs.msdn.com/b/chuckw/archive/2013/08/21/living-without-d3dx.aspx
The legacy SDK uses a lot D3DX and it has not been update since June 2010, so it doesn’t cover and support DirectX 11.1 and beyond.