Definition
A bill of materials is the structured list of everything that goes into a product, broken down part by part and often material by material. It usually mirrors how the product is built, with assemblies, sub-assemblies and individual components, so each line can be traced to a real piece.
The BOM is where substance compliance starts. You cannot assess a part you have not listed, so the completeness of the BOM sets a ceiling on the quality of every check that follows. A forgotten connector or an unlisted coating is a substance risk nobody looked at.
Each line in the BOM is the hook for supplier data. A material declaration, and in the deepest case a full material declaration, is collected against BOM items and then rolled up to the whole product. The assembled result becomes part of the technical documentation that backs a compliance claim.
Compliance is only as good as the BOM under it. A gap in the list is a gap in the assessment.
Note: general educational information, not legal advice. Check the official source before relying on it.