- Custom-made clothing
- Items as part of a bundle
- Used electronics
- New books for pre-order
Explanation: 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