Final choice between contextual and private bid on client/browser side

[yanushP]

In this document bidding_auction_services_system_design.md you mentioned that Seller's ad service can response with two bids (contextual and private).

The response from seller ad service would include a contextual ad winner and / or encrypted Protected Audience response.

It looks like that you want to say that Seller's ad service can response with two bids and then client or browser can choose (run some kind of auction) between contextual and private bids and render the winner.

However, I didn't find any documentation about how this chose/auction of final winner should be done. This doc bidding_auction_services_system_design.md](https://github.com/privacysandbox/fledge-docs/blob/main/bidding_auction_services_system_design.md#sellers-ad-service) doesn't say anything about this scenario.

1. Can you please clarify this part? How should client or browser choose the final auction between contextual and private bids? What info do we need to provide in contextual bid (like bid_price/cpm, etc)?
2. If the contextual bid was chosen, is it required to use Fenced Frame, even-level reporting and other Fledge's technologies or not and we just can run our current stack for rendering and reporting?

[JensenPaul]

I believe it's intended to work the same as on-device auctions: the seller's JavaScript on the publisher's page chooses between a contextual ad and a Protected Audience ad based on the return value from runAdAuction. As the Protected Audience explainer says: "If the auction produces no winning ad, the return value can also be null, although this non-opaque return value leaks one bit of information to the surrounding page. In this case, for example, the seller might choose to render a contextually-targeted ad." I talked more about this in this comment.

If the contextual bid is chosen, it is not required to use Fenced Frames and can be rendered and reported as it is in a world without Protected Audience.

Side note: We are adding optional support for having contextual bids to participate in Protected Audience auctions in #319 in which case they would render and report like other Protected Audience bids which helps avoid the "leaks one bit of information to the surrounding page".