For every product, you must choose a single record reference which will uniquely identify the Information record which you send out about that product, and which will remain as its permanent identifier every time you send an update. It doesn’t matter what reference you choose, provided that it is unique and permanent. This record reference doesn’t identify the product – even though you may choose to use the ISBN or another product identifier as a part of your record reference – it identifies your information record about the product, so that the person to whom you are sending an update can match it with what you have previously sent. It is not recommended to use a product identifier as the whole of the record reference. A good way of generating references which are not part of a recognized product identification scheme but which can be guaranteed to be unique is to prefix a product identifier or a meaningless row ID from your internal database with a reversed Internet domain name which is registered to your organization (reversal prevents the record reference appearing to be a resolvable URL). Alternatively, use a UUID.

This field is mandatory and non-repeating.