Hi,
DIEN material type designed to use Sales use only but you can DIEN material type for purchase too to avoid to use external service management where no need of item category D in purchase order.
Service is an intangible material and as service cannot be stored- so a material master does not contain inventory data for this material type. For more refer use of DIEN
http://scn.sap.com/message/10438511
Regards,
Biju K