A bill of material is made by creating links between items. A bill of materials is a list of links making references to items (parts, documents,...)
Attaching a child part to a part
Attaching a child part to a part is the basis of building a bill of materials. At first this link can seem simple. If you directly attach a part to another, you can add information on the link to caracterize this relationship. But Bill of materials are more complex. When for example we have a car which contains 4 wheels. There could be a single relationship with a quantity information equal to 4. But as we might want to differentiate some information about each instance, we need to link the instance to the relationship.
Attaching a document to a part
attaching a document to a part is more straight forward and goes through a simple relationship