Definition
A BOM describes what an object is made of as a hierarchy: the plant consists of these units, each unit of these parts. In the energy sector it is used for two things that look alike but are not. The engineering BOM says what a plant is supposed to consist of - used for procurement and construction. The maintenance BOM says what it actually consists of now, after twenty years of replacements. The latter is what matters when something fails, and it is also the harder one to keep right: every repair that goes unrecorded widens the gap between the two. The term also appears in software, where an SBOM lists which libraries an application contains - same idea, same reason: you cannot assess risk in something whose contents you do not know.