The additive Snow feature is an experimental component that you can attach to any meshes to add procedural snow to them:
Snow Mat: The material to attach to the procedural mesh
IsAuto: Is the area covered by snow calculated procedurally based on original face normal, if this is set to false the calculation will take the original mesh vertex color to render the snow area so make sure to have painted you mesh before disabling this option
Angle: The value controlling the procedural calculation of snow area, if IsAuto is disabled you can ignore this value. The higher the value the more faces that look in another direction than upward will have snow
This feature is still in experimental phase and suffers some limitation, one that is very noticeable is the incompatibility with hard-edged meshes:
If your custom mesh has a flat shading like this primitive cube then it won't be able to make a coherent snow coverage, you should only use this feature on completely smooth shading like a sphere or a smoothed out rock. An upgraded version of this feature will be released in the coming weeks ignoring original mesh shading.