They are organized by the shader language they use, for the most part this will hold true across the engines.
DX11 will equate to Unreal Engine 4 and CRYENGINE (HLSL) This includes our PBR-Metallic option as well.
PBR-Specular and Basic will work best in Unity. (OpenGL)
With some small amendments to how you set up your bitmaps you can get them to work in other engines but we try to make sure that you never have to jump through hoops like that.