The Protected Audience API should allow additional bids to be scored by the seller on the server-side instead of within the Protected Audience auction.

Scoring additional bids server side would have the following benefits:

1. Lower latency for the Protected Audience auction because additional bids do not need to be scored on the client.
2. Consistent logic for scoring and filtering contextually generated bids for the seller. Currently, sellers would need to run scoring both server-side (for conventional, contextually generated bids that are not additional bids) and client-side (for additional bids).
3. Removal of the dependency of running the Protected Audience auction on the generation of contextual bids. For cases when the Protected Audience auction could be run in parallel with the generation of contextual bids, including additional bids (e.g. Bidding and Auction services), this could result in latency savings.

Currently, the Protected Audience API requires the seller to propagate the buyer’s additional bids into the Protected Audience auction, where the bids are scored and negative interest group targeting is applied. From a seller’s point of view, the scoring of the additional bids in an on-device scoreAd function seems unnecessary because the additional bids are generated using the contextual information server-side by buyers. The sellers have the information sufficient to score these bids server-side, as part of the conventional ad serving flow.

Based on the discussion in #319, additional bids were first introduced as a concept to support negative interest group targeting for contextually generated bids. But, support for negative interest group targeting should be possible without scoring the additional bids on-device. For example, the flow could look like the following for the seller:

1. The seller computes scores for the additional bids server-side, based on the information about additional bids received from buyers.
2. The seller passes additional bids along with their score and other fields specified in the PA API spec to the browser.
3. The browser will apply negative interest group targeting to additional bids, and run the Protected Audience auction to select the final winner.

Would it be possible for the Protected Audience API to be updated to support server-side scored additional bids?