The Permutive ID is a user ID that is set by the SDK and set in First-Party context. PXID is a Third-Party cookie, set by Permutive, which is used to identify users across multiple domains owned by a given Publisher.
This ID is the same within a domain (not publisher-specific). The Permutive ID exists in all environments, including 3P cookie blocked environments like Safari and Firefox. The ID is persistent for a user as long as the local storage isn’t refreshed on the user's device.
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