AppNexus identifies visitors with a 3rd-party cookie set on the appnexus domain ("adnxs.com"). When AppNexus exchanges data between the visitor browser and their server with the tag, the cookie is automatically picked up and passed along with the call because the cookie and tag reference the same domain. So identifying the visitor is done automatically.
However, when the tag logic moves server side, we still need a method to identify who's the visitor in a way that AppNexus understands. This is the purpose of the cookie sync tag. Basically, the tag will call out to AppNexus declaring that we are Tealium and please return the visitor's ID so we can store it as part of their visitor profile.
Then, when the AppNexus action is configured within the UDH, you can configure
Who the visitor is (via the cookie matching tag)
Which segment to add the visitor to
And viola, Appnexus can accept the update and apply it to the appropriate visitor.
Hope this helps!
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