Item sales invoices

I'm preparing a application development specification for a client to get item sales invoices...

In viewing the Endpoint for item sales invoices the lines array does not contain a backorder field. 
The client needs to print backorder information on documents as now happens when printing a MYOB Invoice Form...

Is anyone able to point me to where this information is located...  I can see an Order field on the item sales invoice but the documentation seems to suggest that this is the Order Number from an Order that is being converted to an invoice and is not a relationship to a backorder when entering a new invoice.

I'm concerned that the API does not allow for backorder information to be made available and that Accountright contains a hardcoded solution to print backorder information on the current Invoice Form 

 

If this backorder information is not available the client's requirements will not be able to be met ..

 

Thanks John