The attribute ‘identifier exists’ and value ‘FALSE’ should be used for:
- A) Items as part of a bundle
- B) Used electronics
- C) New books for pre-order
- D) Custom-made clothing
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?hl=en