This code system http://hl7.org/fhir/message-events defines the following codes:

CodeDisplayDefinition
CodeSystem-expandCodeSystem-expandThe definition of a code system is used to create a simple collection of codes suitable for use for data entry or validation. An expanded code system will be returned, or an error message.
MedicationAdministration-CompleteMedicationAdministration-CompleteChange the status of a Medication Administration to show that it is complete.
MedicationAdministration-NullificationMedicationAdministration-NullificationSomeone wishes to record that the record of administration of a medication is in error and should be ignored.
MedicationAdministration-RecordingMedicationAdministration-RecordingIndicates that a medication has been recorded against the patient's record.
MedicationAdministration-UpdateMedicationAdministration-UpdateUpdate a Medication Administration record.
admin-notifyadmin-notifyNotification of a change to an administrative resource (either create or update). Note that there is no delete, though some administrative resources have status or period elements for this use.
communication-requestcommunication-requestNotification to convey information.
diagnosticreport-providediagnosticreport-provideProvide a diagnostic report, or update a previously provided diagnostic report.
observation-provideobservation-provideProvide a simple observation or update a previously provided simple observation.
patient-linkpatient-linkNotification that two patient records actually identify the same patient.
patient-unlinkpatient-unlinkNotification that previous advice that two patient records concern the same patient is now considered incorrect.
valueset-expandvalueset-expandThe definition of a value set is used to create a simple collection of codes suitable for use for data entry or validation. An expanded value set will be returned, or an error message.