The correlation ID is a unique 36 character long ID (e.g. a7c638e2-a1c6-44c7-abfb-807025150f71) that appears in both Search and Redirect Reports when enabled in the subscription manager. As the name suggests, it helps Partners to simply correlate the relationship between search and redirect volumes to uncover how well-served a market or route is.
Is Correlation ID available for historical reports?
The correlation ID is available in historical reports after the date it was released i.e. 20 Aug 2019 onwards. You will have to enable Correlation ID in the subscription to get new historical reports generated with the CID.