Select the family category based on how the family is classified in the industry, that is, how the part is ordered from a manufacturer. Typically, a Revit project will include a schedule of related parts organized by type of manufacturer.
For example, say you are creating a desk for an office environment. If this type of desk would be purchased from a system furniture manufacturer, then the family should be of the System Furniture category. The desk will schedule with the related items to be purchased from that type of manufacturer.