Which is Amazon's recommendation regarding a line item's supply sources?

Amazon recommends separating line items based on their supply source (e.g., Amazon O&O, third-party exchanges) to allow for more granular control and optimization. This approach enables advertisers to tailor bidding strategies, creative assets, and targeting parameters to the specific characteristics and performance of each supply source. For instance, Amazon O&O supply often has unique audience insights and inventory, which might warrant different treatment than third-party exchange inventory. Using all available supply sources for each line item, while simpler, can dilute optimization efforts and make it harder to identify which supply sources are driving performance.