Materials can be linked to other materials in a parent-child relationship called “subcomponents”. As an example, you might have the “Panel” as the parent and the “Glass” and “Frames” as the subcomponents or children. You can access these subcomponents and make changes to them by selecting the greyed out tab.

Subcomponents can be found in a drop down, when searching for materials, as shown in image 1 or on the “related” tab when you click on a material, shown in image 2.