Lian,
When you check this Material Type KMAT (Tcode OMS2) which is assigned to Item Category Group 0002.
At the time of Determination of Item Category you'll get the TAC by the combination of
Document Type + Item Category Group + Usage + Higher Level Item Categoy
Considering your document type as OR
OR + 0002 = TAC
So kindly use Item Category Group 0002 at the time of creation of Customization Material.
Sriram.