This tab displays all the materials loaded from the input file. You can click on a material name to expand its panel and see and edit all of its settings.
Alternatively, if you hover on the preview on the left, you will see the cursor turn into an eyedropper. Click on a material in the preview to automatically expand its card on the right. This is especially useful if the materials were not named correctly in the original model, and you have trouble deciding which is which.
Transmutr tries to automatically retrieve as much information as possible from the input file. So if the input file has information about material reflection, bump or opacity, Transmutr will automatically preserve these settings. You can then edit any setting for any material.
Materials will automatically be exported as regular SketchUp materials (simple color/texture and opacity), V-Ray materials, Thea materials and Enscape materials. It is transparent to you, there's nothing you need to do.
Please note that Transmutr is not intended as a full-featured material editor. Rather, it's here to simplify the process of converting materials when importing assets to SketchUp. If you need more control than what Transmutr has to offer, you should use the material editor of your preferred rendering engine.