Hi Vijay,
My understanding is that internal order is an integral part of the vehicle in case of DBM, the group internal order which gets generated as part of the vehicle creation at DBM itself help to collect all the vehicle cost at vehicle level / batch level.
You can view the VMS vehicle history for action QIOG where in you can see the document flow to the group internal order.
My understanding is that it seems like the same internal gets updated in the vehicle order accounting document assignment details records as well. This is done through the billing create event in the DBM order.
I might validate my understanding & come back.
Regards,
Sachin Balmiki