Here you can enter a document number that refers to the numbering system of the customer or vendor with which you are trading the items on this journal line. You can enter a maximum of ten characters, both numbers and letters.
You can record either a vendor's invoice number or a customer's order number that is associated with the items on this line.
You can then use the number later if, for some reason, you need to search for the posted journal line using the customer's order number or the vendor's invoice number.