The Engineering BOM form displays the relationships between engineering items. You will see the engineering items that are part of the engineering item selected on the Engineering items form.
In principle, the BOM of engineering items is the same as the BOM in the CAD or PDM application when you copy data from the CAD or PDM application to the engineering items. You can, however, manually add BOM lines, for example for packaging material that must be included in the product delivery but is not drawn in the CAD document. The External check box shows whether BOM lines were created either manually, or automatically using the CAD integration functionality.
You synchronize the engineering item with a BOM with a part or a production file. The items copied and their copy destination depend on the settings in the Synchronize section of the Engineering items form.