Check Item Category Groups
Using the item category group, you group
together different material types for item category determination from the SD
view. For every material type, you can define a default item category group
which is proposed by the SAP System when you create a material master record.
For more information on setting default values please refer to the section 
"Define material type defaults.
In the sales document, the system determines
the item category from the item category group of the material and from the
sales document type and proposes this item category in the document. You
assign item categories to sales order types and item category groups
elsewhere. You can find further information on this in the section
"Assign item categories".
Actions
If you define your own item category groups, the keys should start with the letter Z, since this name convention is kept free in the standard SAP System for this purpose.