To flag an appointment, you can use the Flag Icon in the Actions.
This is displayed to the right of an appointment listed in the Consult Workflow:
It is also displayed when viewing the Appointment Details.
When an appointment is flagged, the Flag Icon is highlighted in red to the right of the appointment in the Consult Workflow.
You filter appointments to display all those that are flagged. See help instructions for Filtering Appointments for a guide on doing this.