The Bill of Materials (BOM) is a table that contains information about the parts in a drawing.
You can use the BOM to generate parts lists and balloons.
A drawing file can have several BOMs. The “main” BOM contains information about all the parts and assemblies in a drawing. A parts list generated from the main BOM contains all the parts and assemblies in that drawing.
You can also create a “border” BOM, which contains only the parts and assemblies within a drawing border. Parts lists generated from a border BOM do not contain parts from elsewhere in the drawing.
If mechanical structure is enabled, you can create an Assembly BOM for any assembly in the mechanical structure. An Assembly BOM contains the information of all parts within that assembly.
When a BOM is created, all Part References, parts, and assemblies (components) are included in the BOM automatically. If you then insert more Part References or components to the drawing, the BOM updates automatically.
Because parts lists and balloons are derived from the BOM, the BOM must exist before you can use those features. If you begin to create the balloons or parts lists before you create a BOM, the system automatically creates the BOM for you. If you intend to have multiple BOMs in a drawing, it is easier to create the BOM first rather than create BOMs on the fly along with balloons and parts lists.