This code system http://hl7.org/fhir/participationstatus defines the following codes:

CodeDisplayDefinition
acceptedAcceptedThe participant has accepted the appointment.
declinedDeclinedThe participant has declined the appointment and will not participate in the appointment.
tentativeTentativeThe 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-actionNeeds ActionThe participant needs to indicate if they accept the appointment by changing this status to one of the other statuses.