Where does header bidding lie in the publisher inventory waterfall?

Header bidding is a significant advancement in the digital advertising landscape that allows publishers to offer their inventory to multiple demand sources before making calls to their ad servers. In this process, a request is initiated through the header bidding code embedded within the publisher's website. This approach allows all participating advertisers to bid on the publisher's inventory in real-time, facilitating a competitive auction where the highest bid wins.

Given this context, the option regarding the request sent by header code accurately captures the initial step in the header bidding process, where the publisher's inventory is made available to multiple bidders prior to the conventional waterfall mechanism starting. This effectively improves yield management for publishers, as it enables better price discovery and maximizes the revenue potential from their inventory.

The other options pertain to stages and actions that occur further along in the ad serving process, such as interactions with ad exchanges or actions taken after bids are submitted, which do not represent where header bidding fits within the inventory waterfall itself.