• 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