Definition
The list of every item, component or raw material required to make one finished product; it is the common basis for MRP requirement explosion, backflush and cost accounting.
In detail
One wrong level in a bill of materials miscalculates purchasing requirements upward and cost downward, and neither symptom looks like a bad bill. It is paired with the engineering change notice: if the bill is not updated after a change, the system keeps ordering against the old structure. Pull does not retire the bill of materials — kanban governs when and how much to replenish, while the bill still states what a product is made of, and backflush deducts inventory against it. To judge whether a bill is usable, compare it with how the product is actually assembled, not with whether it exists in the system.
Example in practice
A finished product still lists a seal superseded by an engineering change, so purchasing keeps buying it. A count reveals three months of dead stock, and the team makes updating the change notice and the bill two mandatory steps of the same procedure.