This trigger event requests that a service or resource be removed from an existing scheduled appointment that has not yet begun. A cancel event is used to stop a valid service or resource from participating in the appointment. For example, if a portable X-ray machine scheduled for an exam is no longer needed, then the placer application requests that the resource be canceled on the filler application. This trigger event should only be used for appointments that have not been completed, or for parent appointments whose children have not been completed. If it is successful, an application acknowledgment is returned, optionally containing an SCH segment and related detail segments describing the modified appointment.