Metadata Extension (MDE)
What it is
A separate DDLX object that carries a CDS view's UI annotations outside the view definition, so layout metadata can be changed and layered (e.g. by extension) without editing the view itself.
Why it matters for Clean Core
It keeps annotation-heavy UI metadata out of the data definition and makes the @UI layer separately maintainable and extensible — important for clean, layerable Fiori Elements apps.
Common pitfall
Inlining all @UI annotations in the view 'for now'. Once a view has a serious UI surface, the annotations dwarf the model. Move them to a metadata extension early.