S2s postback is a server-to-server postback, the reverse transfer of data from server to server. In the context of working with PeerClick, this is a way to transfer information about a completed conversion to another platform.
To register a conversion in the affiliate network → PeerClick → traffic source scheme, you need to transfer two postbacks: one from affiliate network to PeerClick and another one from PeerClick to the source.
Postback 1: Registration of Conversion in PeerClick (Affiliate Network → PeerClick)
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 PeerClick. The conversion is displayed in the affiliate statistics and in PeerClick statistics.
With conversion data additional data can be transmitted in the postback - payout amount, conversion status, transaction ID.
More about standard postback from an affiliate network to PeerClick https://help-center.peerclick.com/en/articles/6643019-setting-up-a-postback-to-peerclick
More about probabilistic postback from an affiliate network to PeerClick https://help-center.peerclick.com/en/articles/8425820-probabilistic-postback
Postback 2: Sending Conversion Data from PeerClick (PeerClick → Traffic Source)
After recording the conversion data in PeerClick, PeerClick sends the conversion data to the traffic source.
The conversion is displayed in the affiliate network statistics, in PeerClick statistics and in the traffic source statistics.
More about postback from PeerClick to a traffic source https://help-center.peerclick.com/en/articles/6643122-setting-up-a-postback-from-peerclick