What is the identifier_exists tag for ?
Google must be able to uniquely identify a product. At this end, your products must have at least 2 of the following 3 "unique product identifiers" filled:
Unique product identifiers define the product you're selling in the global marketplace. They uniquely distinguish products you are selling and help match search queries with your offers. Unique product identifiers are assigned to each product by the manufacturer, so if you sell the same product as another retailer, the UPIs will be identical.
However, if your product misses 2 of these 3 previous unique product identifiers, as it's for example the case for manufactured products, ancient books, or other special cases, you can include an "identifier_exists=false" tag which will tell Google that your product simply does not have these references available because of their nature. Google will then examine your catalog and will usually decide to accept the concerned products anyway, as long as you are not abusing this on products that should have these references indicated in your catalog.
To know which unique product identifiers are mandatory for products you're selling or if the "identifier_exists=false" tag will be enough for your feed to be accepted, click here.
As soon as 2 of the 3 unique product identifiers aren't available for a product, our module will automatically include the "identifier_exists=false" tag in the data feed. So you don't have to worry about including it. You just have to make sure this tag is logical for your sales area...