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