Matched on: [[Appointment.External Data Point ID]] as derived from SCH-1.1
When matched: [[Appointment.Status]] set to Cancelled. The other details of the appointment are not updated.
When not matched: New [[Appointment]] created (and immediately cancelled). In this scenario the S15 message will need to conform to the S12 specification to first create the [[Appointment]]. If it does not then PKB will respond with an AR rejection message.