EMF Facet is a solution to extend existing Ecore meta-models without having to modify them.
The idea is to provide non-intrusive mechanisms to add new features (types, attributes, operations and references) to a meta-model and to customize the existing features by adding extra properties (icons, labels, etc.)
For example, EMF Facet is used by :
MoDisco, to adapt source code meta-models to specific use cases (eg. Java code pattern matching);
Papyrus to present UML models in tabular views.