Hi
If you know the IDoc no then go to WE02 transaction and check the control segment of the Idoc.
You will get the basic type of the Idoc.
In WE02, check the data segments and can understand how each value on the Data segments are mapped to FB03.
Now populating the IDoc, how this Idoc is populated ? Do you have scope of populating the Idoc segment OR you want to write enhancement to update the details while Idoc is being processed ? Please let us know.
If you want to populate the segment you can check each segment in WE31 . Hope in segement E1BPACGL09, the field ALLOC_NMBR is the Assignment field. Please check this field if you can post through WE19.
If you want enhancement, the check for substitution ( transaction OBBH & GGB1) / BTE ( transaction FIBF) as per your requirement.