This happens when a particular retailer wants to update the quantities, items etc of an existing order. By the time they send this change order the original order has gone through the process. So when the customers receive an email with the order numbers, and changes that should be applied they need to decide how it can be applied.
1. They can re-import the whole order, and it will import to the A/C system as a new invoice with the changes.
2. Sometimes the users are in the process of pick packing the existing invoice and they don't want to remove the invoice. In that scenario they can update the quantities in the invoice manually and process the order, so that it will bring back the changes.