Brian is expanding his store to include antique goods. What should he do if there isn’t a unique product identifier available?
- Leave all unique product identifiers blank
- Add ‘unique’ to the ‘brand’ attribute
- Use the exemption attribute identifier_exists
- Include the word “antique” in the titles of all products
In the rare situation where your product has neither a GTIN nor MPN (for example, made-to-order or handmade items), set ‘identifier exists’ to ‘FALSE’ to indicate that you have no identifier for the product.
Read more here: https://support.google.com/merchants/answer/160161?hl=en