S2S (Server-to-server) postback URL works on the principle of passing information about the conversion between the server of an affiliate network and the tracker through this link, i.e. GET method. Pixels on the other hand, even though they include the URL, pass information through the cookie of a user's browser. S2S is the more precise and reliable method of tracking conversions, however, there is not always an opportunity to use it. For other cases, the pixels will work.
What is the difference between Tracking pixels and S2S postback URL?
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