The bill of materials is the structured definition of what goes into a product. Planning, costing, purchasing and traceability all inherit its errors.
Single-level and multi-level
A single-level BOM lists the immediate components of one item. A multi-level BOM shows the full tree down to purchased parts. MRP explodes level by level, so intermediate levels should exist where you actually stock or plan a subassembly — not simply because engineering drew it that way.
Engineering versus manufacturing BOM
The engineering BOM describes the product as designed. The manufacturing BOM describes it as built: phantom assemblies, consumables, packaging, scrap factors and the sequence the floor actually follows. Small manufacturers frequently try to plan from an engineering BOM and struggle.
Structure decisions that matter
- Where to stock subassemblies versus flatten them.
- Whether to use phantoms for transient assemblies.
- Scrap and yield factors at the right level.
- Consistent units of measure and conversion.
- Effectivity and revision control.