Usage options for a shader map. The purpose of EMaterialShaderMapUsage is to allow creating a unique yet deterministic (no appCreateGuid) Id, For a shader map corresponding to anyUMaterialorUMaterialInstance, for different use cases. As an example, when exporting a material to Lightmass we want to compile a shader map with FLightmassMaterialProxy, And generate aFMaterialShaderMapIdfor it that allows reuse later, so it must be deterministic.