The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. If the element is present, it must have either a @value, an @id, or extensions Code that identifies the event this message represents and connects it with its definition. Events are defined by codes, or by URIs that identify events, or by reference to EventDefinition or SubscriptionTopic resources that define the event directly. This element is intentionally unbound, and the selection of appropriate event codes is delegated to implementations and implementation guides. For codes, a good set of example codes is found in [the V2 event codes](https://terminology.hl7.org/ValueSet-v2-0003.html). The destination application which the message is intended for. The source application from which this message originated. Coded indication of the cause for the event - indicates a reason for the occurrence of the event that is a focus of this message. Information about the message that this message is a response to. Only present if this message is a response. The actual data of the message - a reference to the root/focus class of the event. This is allowed to be a Parameters resource. Permanent link to the MessageDefinition for this message. The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. Indicates where the message should be routed. Human-readable name for the target system. Allows data conveyed by a message to be addressed to a particular person or department when routing to a specific application isn't sufficient. The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. Identifies the routing target to send acknowledgements to. Human-readable name for the source system. May include configuration or other information useful in debugging. Can convey versions of multiple systems in situations where a message passes through multiple hands. An e-mail, phone, website or other contact point to use to resolve issues with message communications. Identifies the sending entity to allow the use of a trust relationship. The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. The Bundle.identifier of the message to which this message is a response. Code that identifies the type of response to the message - whether it was successful or not, and whether it should be resent or not. Full details of any issues found in the message. OK Transient Error Fatal Error If the element is present, it must have either a @value, an @id, or extensions