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 a 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.).