Bill of materials contains all the parts and assemblies required to build a complete and shippable product. This may include any cobination of items. It also incorporates items that are used in the assembly process, like liquid adhesives or tape. Both off-the-shelf (OTS) components and custom, made-to-specification (MTS) parts belong on a manufacturing bill of materials, as well as non-tangible items like firmware. Any item that can be found in the final boxed product needs to be included at some level of the manufacturing BOM.

Some parts require processing—like pad printing, painting or programming—before they are ready to be assembled into a final product. While only the altered part is assembled into the final product, both the pre-processed base part and the finished part are represented on the manufacturing BOM. The manufacturing team needs to know about all the processing steps in order to make critical decisions about which steps will be performed in-house and which will get outsourced to a separate vendor. The location of the processing may be changed during the life of the product to reduce costs, improve quality or increase flexibility.