This is a Conceptual Data Model for Bill of Materials, which is a hierarchical structure.
This Model shows that each entry in the Hierarchy can have a 'parent' which is
another entry in the Hierarchy.
This is indicated as a 'rabbits ear' or associative relationship.
Here's a version for a Paper Manufacturing facility with a Warehouse
And here is our Toilets Data Model, which uses this BOM Model as a Design Pattern.