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Bidstream data
Bidstream data is the information sent out in real-time bidding requests: the page, the device, the location and visitor identifiers.
Every bid request carries details buyers use to decide what an impression is worth, and every company that receives the request sees them — whether it bids or not. Collected at scale, that stream becomes a dataset of who visited what and where.
That is why bidstream data is a privacy concern and a regulatory target, and why contextual advertising, which targets the page rather than the person, has made a comeback.