Side Effect (RAP)
What it is
A BDEF declaration that tells the UI which fields/entities to re-fetch after a change, so the Fiori app refreshes derived data immediately. Pure UI-freshness metadata — it doesn't change data itself.
Why it matters for Clean Core
Side effects are what make a determination's result appear instantly in the running app without a manual refresh. Declaring them is part of delivering a polished Fiori Elements experience from RAP.
Common pitfall
Forgetting them, then debugging 'why doesn't my calculated total update on screen?'. The determination ran server-side; the UI just didn't know to re-read it. Declare the side effect from the changed field to the derived field.