email_message Allows e-mail messages to be properly defined with all necessary variables and information.
event_message Creates and returns a message_event object, which is used to send direct and RPC calls via RabbitMQ and other event dispatches used within Apex.
event_response Creates a EventResponseInterface object, which is used to send back to the front-end software during a RPC call. This is the response created and returned by the RPC listeners.
sms_message Defines a SMS message all all properties of it.
websocket_message A web socket message interface class.