Today - sellers have no easy way to attach seller information to each additional bid or to differentiate an additional bid within scoreAd or reportResult. Seller-attached information for additional bids could be used to exempt additional bids from on-device eligibility filtering (since additional bids are selected during the server-side contextual ad request, and can thus be subjected to server-side filtering), to compute desirability server-side, and to pass additional bid reporting information through to reportResult. While sellers could attach additional bid data by creating a fingerprint of each additional bid and looking up the matching data in sellerSignals, this proposal is significantly simpler, more efficient, and does not introduce any new privacy concerns.

We propose a generic mechanism for sellers to attach arbitrary seller information to buyers' additional bids via header. Sellers would have access to their sellerData blob for a given additional bid within a new field of the directFromSellerSignals argument to scoreAd and reportResult, directFromSellerSignals.additionalBidSignals.

const signedAdditionalBid = {
  // "bid" from buyers JSON.stringify(additionalBid).
  "bid": "{\"interestGroup\":{\"name\":\"campaign123\"...},...}"
  // Signatures from the buyer of the above “bid” string.
  "signatures": [{
    "key": "ABC123=",
    "signature": "DEF456=="
   }, … ]
  // Proposal: A new seller JSON blob for additional bids.
  “sellerData”: “{ some blob }” 
}

// Signed additional bids are returned via header on the seller’s contextual ad response.
Ad-Auction-Additional-Bid: 
    <auction nonce>:<base64-encoding of the signed additional bid>