The application layer and the domain layer communicate through Message components, which send messages to domain objects. The set of message components and their messages is an API through which domain experts and developers cooperate, and it provides a live context for negotiating domain-object services. The component’s dialog is opened to select among the messages available to the user; it exposes only those messages that can be sent to the object at the Message Receiver sink connector. This message repertoire is determined by a class dictionary for the class of the domain object at the Message Receiver input.