Bill of Materials Tab Reference (Document Settings)
Specifies bill of materials (BOM) settings for the selected component.
Access
Ribbon: Tools tab 
Options panel Document Settings 
. In the dialog box, click the Bill of Materials tab.
The active document type determines the available options.
Default BOM Structure
Sets the default BOM structure for the component.
Note: You can override the structure to Reference on any component instance levelling an assembly.
Unit Quantity
Displays the Unit Quantity, which is composed of base quantity and base unit properties.
Base Quantity
Sets the base quantity for the component. Select a parameter to use as the base quantity, and click OK. To edit or add a parameter, click Edit Parameter next to the Base Quantity option. The Base Quantity option is read only for assembly documents in Inventor.
Note: If the parameter that the base quantity is assigned to is deleted, then the base quantity is reset to Each.
Base Unit
Select a base Unit from the list, and click OK. The Base Unit option is read only for assembly documents in Inventor.