#276 proposes adding/updating IG priority in the tagging call, and sellers can use perBuyerGroupLimits to control the number of interest groups for each buyer participating in an auction. There has been some insightful discussion about modifying IG priority based on trustedBiddingSignals (see e.g. #79). Our analysis shows that leveraging trustedBiddingSignals can significantly improve utility. The trusted server can prune some IGs, for example when their corresponding ads are out of budget. Other valuable features like geo, domain, and ad features can help to provide a more accurate prediction on how likely the IG will win.

To be more concrete, we propose that,

1. Trusted server is allowed to specify a trustedBiddingInterestGroupPriority and include that as one of the real time bidding signals. Then generateBid is invoked according to the ranking determined by trustedBiddingInterestGroupPriority. The generateBid execution can be terminated once we reach the cap of IG set by perBuyerGroupLimits for a certain buyer, or more reasonably until hitting the total execution time allocated to that buyer (proposed in #293).
   • More optimizations can be implemented, for example to directly drop the interest group if the trustedBiddingInterestGroupPriority is 0.
2. The trusted server IG priority ranking can be combined with the tagging time IG ranking to form a two-stage selection process,
   • In tagging time, determine a preliminary IG priority, pre-select a relatively larger number of potential IGs.
   • trustedBiddingInterestGroupPriority is a more accurate score and can be used to determine the sequence of generateBid calls.
3. A more efficient way, in terms of payload size and latency, is to allow the trusted server to look at all IGs within the same request and select the top K. In this case there is no need to pre select IG during tagging time.