Setting up a bill of materials involves assigning material items in your standard database to material classes. As you set up your bill of materials, you specify:
After you assign an item to a material class, the bill of materials information is included with the item during takeoff.
Example: Your standard database may contain multiple 3000 psi concrete items that you want to print as one line item with the supplier's description, "Regular Concrete 3000 psi."

Here are some links to help you get started:
Setting up a bill of materials (SQL)
Setting up a new material class (SQL)
Changing a bill of materials (SQL)
Changing an existing material class (SQL)