A facet represents configuration specific for a particular framework/technology, associated with a module. A module can have multiple facets. E.g. Spring Framework specific configuration is stored in a Spring facet.
Facet Basics Sample
Please see Facet Basics sample plugin project.
Working with Facets
To create, search and access the list of facets for a module use
Facet-Based Tool Window
A tool window dependent on the existence of given facet(s) can be registered via
com.intellij.facet.toolWindow extension point.