S2s postback is a server-to-server postback, the reverse transfer of data from server to server. In the context of working with a tracker, this is a way to transfer information about a completed conversion to another platform.
To register a conversion in the affiliate network → tracker → traffic source scheme, you need to transfer two postbacks: one from affiliate network to the tracker and another one from the tracker to the source.
Postback 1: Registration of Conversion in the Tracker (Affiliate Network → Tracker)
When a conversion is made, it is recorded by the affiliate network. The conversion is displayed in the affiliate network statistics.
Then the conversion data is sent with the postback to the tracker. The conversion is displayed in the affiliate statistics and in the tracker statistics.
With conversion data additional data can be transmitted in the postback - payout amount, conversion status, transaction ID.
More about postback from an affiliate network to the tracker https://help-center.peerclick.com/en/articles/6643019-setting-up-a-postback-to-peerclick
Postback 2: Sending Conversion Data from the Tracker (Tracker → Traffic Source)
After recording the conversion data in the tracker, the tracker sends the conversion data to the traffic source.
The conversion is displayed in the affiliate network statistics, in the tracker statistics and in the traffic source statistics.
More about postback from the tracker to a traffic source https://help-center.peerclick.com/en/articles/6643122-setting-up-a-postback-from-peerclick