When submitting product identifiers, the following should be applied.
If a manufacturer has assigned a bar code, then submit brand, mpn (if available), GTIN, and set identifier exists to yes.
If the manufacturer has not assigned a bar code, then remove the values brand, mpn, gtin, and set identifier exists to no.
While the official documentation does mention, you can submit brand + mpn and ignore GTIN. In most cases, this will cause issues and generate warnings such as limited performance due to missing identifiers. This applied to warnings before June 2021.
As of June 2021, if the manufacturer has not assigned bar codes and your brand, gtin, and mpn values are removed, you will still get the warning “limited performance”. This is because Google now adds another check depending on which categories your products belong to.
Meaning regardless of adding accurate data, you may still get flagged.
What You Can Not Do!
Only submit GS1 barcodes, any other barcodes that you self generate are not allowed and will get your products disapproved.
The most common misconceptions are that most merchants tend to use SKU as the MPN values, which is incorrect.
SKU stands for “stock keeping unit” and — as the name suggests — it is a number (usually eight alphanumeric digits) that retailers assign to products to keep track of stock levels internally.
An MPN, or manufacturer part number is a manufacturer-assigned, unique, alphanumeric value that is used to identify a product among other products from the same manufacturer.
How To Find The Bar Codes?
By checking the product packaging or by contacting the manufacturer.
A secondary reason for issues is that most merchants tend to use SKU as the MPN values, which is incorrect.
Why Is Google Doing This?
Most likely to reduce scammers on Google Shopping.
What If I Can’t Do Any of The Above?
Great question. In this case, you will have to accept the limited performance. If you are the manufacturer, consider registering your products with GS1 bar codes.
Remove All Values & Set Identifier Exist to No
If you want to easily remove all values.
You can use feed rules in Google Merchant Center to remove the values Brand, MPN and GTIN (assuming GTIN has wrong values) then set Identifier Exists to the value of No.
You can find the official guide here: https://support.google.com/merchants/answer/7450276