Publisher first-party IDs are a first-party cookie (on web) or ID (on app) set by Google when third-party cookies or device identifiers are not available. These IDs are scoped to a publisher’s domain or apps, and are not shared with other publishers.
They can be used for interest-based ad personalization, allowing buyers to generate first-party user profiles based on user activity on the publisher's site/apps. These IDs can also be used for frequency capping.
To support publisher first-party ID signals in your OpenRTB integration, refer to the protocol guide.
Publisher first-party ID buyer access
Most publishers share first-party IDs with both Google demand and RTB (Authorized Buyers, Open Bidders, and SDK bidders), potentially giving buyers high coverage access to these identifiers across a wide range of inventory where permitted.
Interaction of publisher first-party IDs with other identifiers
If a publisher supports both Publisher first-party IDs and PPIDs, and both identifiers are available to be sent in an ad request, only one will be sent. Publisher-first party IDs are not passed when third-party cookies or device identifiers are available.
Learn more about publisher first-party identifiers.