You can now add an optional 'additional' reference for your internal or end-user delivery use. The reference is added just after the PO number is added during checkout. This reference will appear on your invoice and proof of delivery directly after the last product and your order information on TDP. 

The reference can accommodate up to 50 characters of numbers, letters and dashes, including spaces; however, special characters are not allowed in this field. 

It looks like this: