Determination
What it is
A RAP behavior that automatically derives or fills data when a trigger condition occurs (e.g. on create, or when a field changes). Declared in the BDEF, implemented in the behavior pool. Think 'derived value', not 'validation'.
Why it matters for Clean Core
Determinations are where derived business data belongs in RAP — so the logic runs consistently whether the change came from the Fiori UI, OData, or EML. It's the right home for the logic that used to live in user-exits.
Common pitfall
Using a determination to reject bad input — that's a validation's job. A determination changes data; a validation accepts or rejects it. Mixing them up leads to determinations that silently 'fix' data the user never intended.