Matched on: [[Appointment.External Data Point ID]] as derived from SCH-1.1
When matched: Partial update. See S12 specification for which fields will be left unmodified if a blank is provided. If the matched appointment has an [[Appointment.Status]] of ‘CANCELLED’ or ‘DNA’ then status will revert to ‘ACTIVE’. Note: the list of external participants is purged and replaced.
When not matched: New [[Appointment]] created.