Cookie syncing
Cookie syncing is the practice by which two advertising technology companies match the different cookie identifiers they each hold for the same browser, so they can exchange data and bid on that user.Each ad tech company can read only its own cookies, so the identifier a demand-side platform holds for a browser differs from the one an exchange or a data broker holds. Syncing joins them. When a page loads a tag from company A, A redirects the browser to company B with A's identifier in the URL; B reads its own cookie from the same request and stores the pair in a match table. From then on, either company can refer to the user in the other's terms, which is what lets audience data and bids flow between them in real-time bidding.
Each step stores or reads an identifier on the device and passes it to another company, so in the EU each one falls under Article 5(3) of the ePrivacy Directive and needs consent, and each recipient needs a GDPR legal basis. Under California law, passing identifiers to a third party for cross-context behavioral advertising is sharing that visitors can opt out of.
Browsers that block third-party cookies by default, such as Safari and Firefox, break most cookie syncing. For a site owner, the practical point is that accepting one advertising tag can bring in many other companies, which is why the IAB TCF lists vendors one by one and why marketing cookies should stay blocked until consent.