A bill of materials or product structure (sometimes bill of material, BOM or associated list) is a list of the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture an end product. A BOM may be used for communication between manufacturing partners or confined to a single manufacturing plant. A bill of materials is often tied to a production order whose issuance may generate reservations for components in the bill of materials that are in stock and requisitions for components that are not in stock. There are two types of bill materials. A BOM can define products as they are designed (engineering bill of materials), as they are ordered (sales bill of materials), as they are built (manufacturing bill of materials), or as they are maintained (service bill of materials). The different types depend on the business need and use for which they are intended. In process industries, the BOM is also known as the formula, recipe, or ingredients list. The phrase "bill of material" (or "BOM") is frequently used by engineers attributively to refer not to the literal bill, but to the current production configuration of a product, to distinguish it from modified or improved versions under study or in test. Sometimes the term "pseudo-bill of materials" or "pseudo-BOM" is used to refer to a more flexible or simplified version. Often a place-holder part number is used to represent a group of related (usually standard) parts that have common attributes and are interchangeable in the context of this BOM. In electronics, the BOM represents the list of components used on the printed wiring board or printed circuit board. Once the design of the circuit is completed, the BOM list is passed on to the PCB layout engineer as well as the component engineer who will procure the components required for the design. BOMs are of hierarchical nature, with the top level representing the finished product which may be a sub-assembly or a completed item. BOMs that describe the sub-assemblies are referred to as modular BOMs. An example of this is the NAAMS BOM that is used in the automotive industry to list all the components in an assembly line. The structure of the NAAMS BOM is System, Line, Tool, Unit and Detail. The first hierarchical databases were developed for automating bills of materials for manufacturing organizations in the early 1960s. At present, this