Service Definition & Service Binding
What it is
The two RAP artefacts that publish a business object as a consumable service. The service definition lists which entities are exposed; the service binding binds that to a protocol (OData V2/V4 UI, OData API) and creates the service URL.
Why it matters for Clean Core
They're the final step that turns a modelled BO into something a Fiori app or integration can call — and the place you control which protocol and which entities you expose.
Common pitfall
Exposing the interface (engine) layer directly instead of a projection through the service definition. Publish the consumption projection so you control the exposed surface and keep the core model private.