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Shader selection is quite simple. Just choose the shader you wish to use from the drop down list. All PBR shaders start with "L2IBL", and you can see more details at the shader definition pages.
Mapper is for the Specular probe assigned to this material. You can leave it blank by default and it will assign to the default probe. If you wish to assign a custom probe then you can do so here. (For example the interior of a garage).
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