All Magento products to Google Shopping needs to have two out of three unique identifiers
Since not all products have MPN or GTIN numbers Google decided to introduced the attribute with the name‘identifier exists’ to allow merchants to sell custom made goods, old books and retailer-specific bundles that don't have GTIN numbers or MPN's.
Use the Product Identifier Exists attribute in the Google Shopping Extension
In the configuration of the extension in System -> Configuration -> Magmodules -> Google Shopping -> Advanced Options -> Identifier Exists
There are 3 options to configure the Identifier Exists attribute;
Identifier exists = FALSE
Now you inform Google that you don't have product identifiers on the products.
Activate when less than two
Identifier exists = FALSE
Now you inform Google that you don't have product identifiers on the products but only on products with less than 2 product identifiers in your feed.
Identifier exists = TRUE
Now you inform Google that you do have product identifiers on the products.
What is the Product Identifier Exists Attribute
The default value of ‘identifier exists’ attribute is always set to TRUE (activated) assuming that the unique identifier exists as Google requires this, therefore, the number should be entered for the product to be included in the Shopping feed. Items where ‘identifier exists’ is set to TRUE but no GTIN or MPN inserted the product will not be submitted to Google Shopping feed.
With products where unique product identifiers are required, merchants must submit the ‘identifier exists’ attribute with a value of FALSE when the item does not have unique product identifiers appropriate to its category, such as GTIN, MPN, and brand.
Examples of how to submit unique product identifiers correctly:
Rare old books: submit ‘identifier exists’ with a value of FALSE.
New books (published after 1970): provide ISBN.
Hand made goods: submit ‘identifier exists’ with a value of FALSE.
Antique goods: submit ‘identifier exists’ with a value of FALSE.
Mass-produced clothing: provide ‘brand’.
Custom clothing: submit ‘identifier exists’ with a value of FALSE.
Manufacturer-defined multipack: follow the Unique Product Identifier Rules as described above. This multipack should be treated as a single item.
Merchant-defined multipack: follow the Unique Product Identifier Rules as described above for the individual base item in the multipack.
A manufacturer-defined bundle (e.g., a camera body with a lens): follow the Unique Product Identifier Rules as described above. This bundle should be treated as a single item.
A merchant-defined bundle (e.g., a custom bundle of camera, lens, and case): submit ‘identifier exists’ with a value of FALSE for these items and do not provide unique product identifiers, even if the bundle contains some individual items that have unique product identifiers.
The ‘identifier exists’ for Google Shopping can be found in the Advanced section of the Google Shopping configuration in your Magento Back-end.