The Shell material is for use with texture baking.
When you use Bake To Texture, it creates a Shell material that contains two materials: the original material used in the rendering, and the baked material. The baked material uses a bitmap that is saved to disk by Bake To Texture. It is “baked,” or attached to an object in the scene.
The Shell material is a container for other materials, like Multi/Sub-Object. It also lets you control which material is used in which renderings.
To load a shell material into the Material Editor:
The sample slot now contains the baked material.
Displays the name of the original material. Click the button to view that material and adjust its settings.
Displays the name of the baked material. Click the button to view that material and adjust its settings.
In addition to the coloring and maps used by the original material, the baked material can include shadows from lighting, and other information. Also, a baked material has a fixed resolution.
Use these buttons to choose which material appears in shaded viewports: the original material (upper button) or the baked material (lower button).
Use these buttons to choose which material appears in renderings: the original material (upper button) or the baked material (lower button).