In the AppointmentResponse resource for FHIR r4 , the comment attribute identifies the Additional comments
"comment":
"identifier":
"appointment": {
"reference": "Appointment/example",
"display": "Brian MRI results discussion"
}
"start":
"end":
"participantType":
"actor": {
"reference": "Patient/example",
"display": "Peter James Chalmers"
}
"participantStatus": "accepted"
"comment":