Campaign priority should be used when:
- A) You want to lower the bid for a product group
- B) You have multiple sales to promote within the same campaign
- C) You want to separate your inventory by brand
- D) You have multiple campaigns advertising the same products
Explanation: Campaign priority is useful when you’re advertising the same product, for the same country, in multiple Shopping campaigns.
Read more here: https://support.google.com/adwords/answer/6275296?hl=en