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If you don't configure tracking domains the tracking domain "go.flx1.com" will be used. After configuring the tracking domain you can select it when creating or updating a tracking pixel.
Please note that cookies are separated by tracking domain. So if you change the tracking domain for existing users or create a new one they won't share the cookies.
This will result in all users getting new DMP UUID's.
https://blog.chromium.org/2020/01/building-more-private-web-path-towards.html
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