A value assigned to a particular user or device and is used to assist in ad serving functionality. The identifier contains no personally identifiable information, can be reset by the user at any time, and respects user options to limit or opt out of ad tracking. Identifiers are used for audience list targeting, frequency capping, and sequential creative rotation. Examples of identifiers are cookies (used on web and mobile web), mobile advertising IDs (used within mobile apps) and publisher provided identifiers (PPID, used for cross screen serving).