The MoDisco facet component introduced the concept of "facet". A facet is a kind of virtual class extension. Those "virtual classes" will never have instances. An object can conform to a facet but can not be an instance of a facet.
For example, assuming we have a class named Employee and two facets: "Manager" and "Developer". We will have a set of Employee instances and some of them will have the facet of "Manager" or "Developer".
The goal of introducing the concept of facet is to be able to highlight some model elements as corresponding to a specific facet.
The MoDisco facet feature allows describing and using facet. The description is done by editing a model and the use is done through an API. The main client of this API is the MoDisco browser.