In this chapter, you surveyed the kinds of Messages (130) your actors can sent and receive and how the intent of each operation determines the kind of Message (130) you will use. You will use Command Message (202) to request an operation to be performed, a Document Message (204) to reply to a query request, and an Event Message (207) to convey that something has happened in your actor system’s domain model. A Command Message (202) and a Document Message (204) will be used together to form a Request-Reply (209). The Actor model always provides the Return Address (211) of the actor to which the reply part of Request-Reply (209) should be sent. You also saw how to leverage a Correlation Identifier (215) to associate a reply with a given request and how you can use Message Sequence (217) when the order of messages to be handled is important. When messages can become stale, use a Message Expiration (218) to indicate the “shelf life.” You also saw how versions of messages can be set by using Format Indicator (222).