See this link. 
https://developer.amazon.com/en-US/docs/alexa/smarthome/smart-home-skill-api-message-reference.html
| Operation | Message |
|---|---|
| Respond to a directive request that completes successfully | Response |
| Request a state report | Alexa.ReportState |
| Respond to a state report request | Alexa.StateReport |
| Proactively notify Alexa that an endpoint property has changed | Alexa.ChangeReport |
| Notify Alexa that you are responding asynchronously to a message because the operation will take longer than 8 seconds. (used for locks) | Alexa.DeferredResponse |
| Report whether an endpoint has connectivity | Alexa.EndpointHealth |