A Bill of Material (BOM) is a formally structured list of the components that make up a product or assembly.
The list contains the object number (number assigned to a component to distinguish them from another) of each component, together with the quantity and the unit of measure.
The purpose of creating a BOM is to specify the material components that are needed to execute a production order.
BOM used for:-
Production – to plan provisions of component
materials
Purchasing – to determine purchase quantities
Costing – to calculate costs of materials
Planning – to determine lower level
requirements
You can use the transaction code CS01 or the following SAP menu path to create
a BOM.
Logistic
> Production > Master Data > Bill of Material > Bill of Material
> Material BOM
Fill up
Material, Plant and BOM Usage
System displays Create material BOM: General Item
Overview screen. In Material tab, system accepts item category,
component, quantity, and UN.
System displays Create material BOM: General Data screen.
Select the item and click the Item button.
System displays Create material BOM: Item: All data screen
System accepts fixed quantity, item number, and the
component
In the Status/Lng Text tab, system accepts Item Text
and ensures that the Item status is set.
Click on the Header button
System displays Create material BOM: Header Overview screen.
In the Quants/long txt tab, system
accepts the BOM text, Alt text, Base Quantity, and BOM status. 
Save
