What is S2S in search arbitrage?
S2S means search-to-search: buying a click from one search ad and sending the visitor to a page with another set of search ads. The buyer pays a low price for a broad keyword and hopes the visitor then clicks a higher-priced ad. In tracking, the same letters also mean server-to-server, another name for a postback.
Search-to-search is the purest and most controversial form. The visitor has already searched once; the arbitrage page asks them to search again. It works only when the first click is cheap, for example on a less competitive engine, keyword or country, and the second is dear.
Search engines watch it closely, because their own advertisers can end up paying twice for the same person, and ad platforms have rules about destination quality that thin search-results pages can fail. Feeds restrict which search sources are allowed. Context usually tells you which meaning is intended: "S2S feed" is the business model, "S2S tracking" is a postback.