How can I add a PO number to an invoice (or multiple PO's) numbers)

A customer may ask a PO number to put as reference on your invoice. The PO number should be added on Order header level in the property Reference.

PO on header

{
 "OrderType": "Invoice",
 "OrderDirection": "Income",
 "OrderNumber": "QS-004",
 "OrderDate": "2025-05-05",
 "ExpiryDate": "2025-05-30",
 "Reference": "45123456789", //PO Reference of the buyer on header level
}

Remarks:

Put only one PO number in this tag.
Only put the PO reference, not additional text.

PO on line

Although **not **the preferred scenario with networks such as Peppol, it might be useful/needed to reference PO numbers on line level (one PO per line):

In that case only one PO is mentioned on header level,
and multiple on line level (one PO per line).

First option is to include PO as text in a LineNote as free text. Benefit is that it is easy to understand for a receiver:

                 {
                "Quantity": 3,
                 "UnitPriceExcl": 5.00, 
                 "Description": "Cookies",
                 "DescriptionExtended": "Box of cookies 20 pieces, 200 g", //Optional longer description
                 "Reference": "917865", //Supplier Article Number
                 "VATPercentage": 0,
                "CustomFields" :
         {
               "InvoiceLine.Note": "Purchase Order Number 45123456789" 
        },                 

   }

Second option is to include PO in commodity classification:

The listID PO is an offical value in the code list.
Not all receivers may be able to process this information. So you can combine with free text in InvoiceLine.Note.
Below example of the combination.

                 {
                "Quantity": 3,
                 "UnitPriceExcl": 5.00, 
                 "Description": "Cookies",
                 "DescriptionExtended": "Box of cookies 20 pieces, 200 g", //Optional longer description
                 "Reference": "917865", //Supplier Article Number
                 "VATPercentage": 0,
                "CustomFields"     :
    {
     "CommodityClassificationPO": "45123456789", 
		 "CommodityClassificationSRV": "5410355405132" ,
		 "InvoiceLine.Note": "Purchase Order Number 45123456789" 
     },                 
   }