The material you select is assigned to the component you select.
(recommended)

While in the Material browser, you can change the material for a component.
Material and Appearance panel
click Materials
.(recommended)
In the QAT, at the bottom of the Material drop-down list, select the library that contains the material. The list updates to reflect the contents of the selected library.
In the graphics area, select a component.
In the QAT, Material drop-down list, select the material to assign to the component.
On the ribbon, click Tools tab
Material and Appearance
Materials
. The Material Browser displays.
In the graphics area, select a component.
In the Library Materials list section, pause the cursor over a material you want to add to the document. The material previews on the selection.
Right-click the material, and click Assign to Selection. This action adds the material to the document, and assigns it to the component.