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

CodeDisplayDefinition
requiredRequiredThe participant is required to attend the appointment.
optionalOptionalThe participant may optionally attend the appointment.
information-onlyInformation OnlyThe participant is excluded from the appointment, and may not be informed of the appointment taking place. (Appointment is about them, not for them - such as 2 doctors discussing results about a patient's test).