Action (RAP)
What it is
A non-standard operation on a RAP business object beyond create/update/delete — e.g. 'Approve', 'Release', 'Copy'. Declared in the BDEF and exposed as an OData function/action; it can change the BO and return a result.
Why it matters for Clean Core
Actions are how you model real business verbs cleanly instead of overloading update logic or building a separate report. They surface as buttons in Fiori Elements automatically.
Common pitfall
Implementing 'Approve' as an update to a status field from outside the BO. Model it as an action so the transition runs the BO's validations and determinations and shows up as a first-class operation in the service.