Also, sometimes called a UUID. A unique, anonymous user ID for a given user profile that may be stored in a user's browser cookie and/or in the ad server-side cookie store. Ad traffickers are not permitted to associate these IDs in any way with personally identifiable information (PII), and user IDs do not necessarily equate to a unique individual. An individual may also be associated with multiple user IDs due to clearing cookies or using multiple browsers and multiple devices. Additionally, for mobile devices, a unique user ID may be associated with multiple device IDs (for example, Apple IDFA, OpenUDID, etc.).