This case-sensitive code system http://hl7.org/fhir/participationstatus defines the following codes:
Code
Display
Definition
accepted
Accepted
The participant has accepted the appointment.
declined
Declined
The participant has declined the appointment and will not participate in the appointment.
tentative
Tentative
The participant has tentatively accepted the appointment. This could be automatically created by a system and requires further processing before it can be accepted. There is no commitment that attendance will occur.
needs-action
Needs Action
The participant needs to indicate if they accept the appointment by changing this status to one of the other statuses.