Projection View vs. Interface View
What it is
RAP's two-layer CDS pattern. The interface view (the 'engine', I_*) holds the reusable data model; the projection view (the 'consumption' / C_*) exposes a tailored, annotated subset for one service. RAP behavior projections mirror this for behavior.
Why it matters for Clean Core
The split keeps the core model reusable while each app/service projects its own fields, annotations and authorisations — the Clean Core layering (I_ for stable reuse, C_ for consumption) SAP itself follows.
Common pitfall
Building one giant view that's both the model and the UI and reusing it everywhere. The first consumer's @UI annotations then leak into every other use. Separate the interface from the projection from the start.