This table stores prior IDs in a generic fashion for use by third parties and/or custom development for syncing people data to MP where finding a new Contact_ID value is required when a previously stored Contact_ID value is no longer in our system.
Records are created when the Combine Contacts Tool is used. Records might also be created by third party integration activity (e.g. Vision2, PushPay, etc.) and/or custom development.
If you cannot find a previous Contact_ID you can search this table (available via REST API) and retrieve the new Contact_ID.