The list of components that make up a system. For example, a bill of materials (BOM) for a house would include the cement block, lumber, shingles, doors, windows, plumbing, electric, heating and so on. Each sub-assembly also contains a bill of materials; the heating system is made up of the furnace, ducts, etc. A bill of materials “implosion” links component pieces to a major assembly, while a bill of materials “explosion” breaks apart each assembly or sub-assembly into its component parts.
The first hierarchical databases were developed for automating bills of materials for manufacturing organizations in the early 1960s.