Key-User Extensibility
What it is
In-app, no-/low-code extension done by a business key user through Fiori tools: custom fields, custom logic (BAdIs via the Custom Logic app), custom CDS views, and UI adaptation. No transport, no developer.
Why it matters for Clean Core
It's the cleanest extension there is — SAP owns the lifecycle, so it survives upgrades by construction. Every requirement a key user can satisfy is one fewer custom ABAP object you have to govern.
Common pitfall
Assuming key-user extensibility is 'too limited to bother with'. Custom fields propagate end-to-end (DB → CDS → OData → UI) automatically, and custom logic BAdIs cover far more than people expect. Check it first.