I can first tell you that backordered lines are not included in the shipment. These lines will be added to a shipment at backorder release.
If by trailer you mean the vehicle number that is going to be used to ship the goods, then you could work with different vehicles, but you would have to use transportation loads. At load confirmation (P49640) you would need to add a vehicle number and you could confirm only the quantity that is going to be loaded on that respective vehicle. The rest of the quantity can then be loaded on a different vehicle and so on.
Please let me know if this answers your questions.
Thanks and regards,
Thank you, this is answering my questions.