Commodity parts are defined on the S.10.03 Commodity Parts screen.
Select the group you have defined for the assemblies in the first (Commodity Groups) block.
Next, enter the part name in the Part field of the second (Commodity Parts) block and the part descriptions in the Short Desc and Description fields. The commodity rule is copied from the commodity group, but you can change the rule here in the Rule field. You can assign a standard to the selected part using the LOV in the Standard field.