A bit more digging. The google_user_id field is described a bit more here: https://developers.google.com/authorized-buyers/rtb/cookie-guide https://developers.g

A bit more digging. The google_user_id field is described a bit more here: https://developers.google.com/authorized-buyers/rtb/cookie-guide https://developers.google.com/authorized-buyers/rtb/cookie-g... From what I can tell it looks like advertisers who already have user IDs can use this in their ad campaign but all google_ fields are stripped from ad redirects.