The components with the little red doors are called “gateway components”. Opening the doors reveals full-service APIs presented by the domain objects. (The APIs in the projector components are also full-service, but are typically a given part of the prototyping environment, so we are ignoring them here.)

Definition: A full-service API gives you everything you need in order to understand it, test it, and connect to it, if you choose to, without ever leaving the API. Importantly, it doesn’t require you to construct grammatical expressions; rather, it presents choices, which it explains.

So it’s gateway components and full-service API’s that extend a simple wiring diagram into something that can describe a whole application.