When you create a family, the template assigns it to a category that defines the default display of the family (line weight, line color, line pattern, and material assignment of the family geometry) when the family is loaded into a project. To assign different line weights, line colors, line patterns, and material assignments to different geometric components of the family, you need to create subcategories within the category. Later, when you create the family geometry, you assign the appropriate components to the subcategories.
For example, in a window family, you could assign the frame, sash, and mullions to one subcategory and the glass to another. You could then assign different materials (wood and glass) to each subcategory to achieve the following effect.
In a plumbing fixture family, you could assign the sink to one subcategory and the fittings to another. You could then assign different materials to each subcategory.
Revit features some predefined subcategories for different categories of families. Other families have no subcategories, which means that you can define your own. The Object Styles dialog lists family categories and subcategories. It also displays the line weight, line color, line pattern, and material assigned to each category and subcategory.
Revit automatically selects the appropriate category in the Subcategory of list.
Although you will not immediately create and assign the subcategory to the family geometry, you can specify the line weight, line color, line pattern, and material for the subcategory.
See Materials.