Automatic GL account pick for service PO

When I raise PO with account assignment u2018 Ku2019 it ask cost center and pick the GL account automatically from OBYC table. But when I raise PO for service with item category u2018 D u2019 and account assignment u2018 k u2018 it picks different GL account. I think that GL account pick which is assigned for service material group. Any one can tell me how and where from that GL account pick. Anywhere else GL account is assigned for service material group?