How the Identifier Exists attribute Works

All Magento products to Google Shopping needs to have two out of three unique identifiers

1. Brand
2. MPN
3. GTIN

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;

Activate
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.

Deactivate
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:

Books

Rare old books: submit ‘identifier exists’ with a value of FALSE.
New books (published after 1970): provide ISBN.

Custom Goods

Hand made goods: submit ‘identifier exists’ with a value of FALSE.
Antique goods: submit ‘identifier exists’ with a value of FALSE.

Apparel

Mass-produced clothing: provide ‘brand’.
Custom clothing: submit ‘identifier exists’ with a value of FALSE.

Multipacks

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.

Product Bundles

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.