The Add button enables you to import additional parts. This can be useful when modeling a family mold.
A family mold contains two or more cavities, and each cavity forms a part to an assembled product. Unlike multi-cavity molds that have multiple identical cavities, family molds have multiple cavities that are not identical.
Once you have added the second part, you will need to orient it correctly, using the Move tools in the Utilities tab of the Geometry tab.