"OrderHeader" entity doesn't has value for billToPartyId and billFromPartyId in S.O and its linked P.O

[sakthi vadivel]

hi folks

                When i am creating a sales order which is of type
"drop-ship only" the linked purchase order does not has value in
"OrderHeader" entity for "billToPartyId" and "billFromPartyId".

I have debugged the code in OrderServices.java->createOrder method
there these fields aren't used.
Still i do not know why this entity doesn't has value for this fields.

All your assumptions and suggestions are welcome.

regards
sakthi