This code system http://hl7.org/fhir/subscription-notification-type defines the following codes:

CodeDisplayDefinition
handshake HandshakeThe status was generated as part of the setup or verification of a communications channel.
heartbeat HeartbeatThe status was generated to perform a heartbeat notification to the subscriber.
event-notification Event NotificationThe status was generated for an event to the subscriber.
query-status Query StatusThe status was generated in response to a status query/request.
query-event Query EventThe status was generated in response to an event query/request.
<status value="active"/><experimental value="false"/><date value="2021-01-05T10:01:24+11:00"/><publisher value="HL7 (FHIR Project)"/><contact><telecom><system value="url"/><value value="http://hl7.org/fhir"/></telecom><telecom><system value="email"/><value value="fhir@lists.hl7.org"/></telecom></contact><description value="The type of notification represented by the status message."/><jurisdiction><coding><system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/><code value="001"/><display value="World"/></coding></jurisdiction><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/subscription-notification-type"/><content value="complete"/><concept><code value="handshake"/><display value="Handshake"/><definition value="The status was generated as part of the setup or verification of a communications channel."/></concept><concept><code value="heartbeat"/><display value="Heartbeat"/><definition value="The status was generated to perform a heartbeat notification to the subscriber."/></concept><concept><code value="event-notification"/><display value="Event Notification"/><definition value="The status was generated for an event to the subscriber."/></concept><concept><code value="query-status"/><display value="Query Status"/><definition value="The status was generated in response to a status query/request."/></concept><concept><code value="query-event"/><display value="Query Event"/><definition value="The status was generated in response to an event query/request."/></concept></CodeSystem>