You can explode a BOM, that is, break it down into the individual items it consists of. If a component in a BOM is a BOM itself (and thus has a check mark in the Bill of Materials field), you can explode it and replace it with its components.
Open the Bill of Materials window.
In the Bill of Materials window, select a component in your BOM which has a check mark in the Bill of Materials field.
Click Explode BOM.
The original component in your BOM is now removed, and the items that comprise it are inserted.
Creating New BOMs
Viewing BOM Ledger Entries