proposition:
FAMIXClass represents an entity which can build new instances. A FAMIXClass is a FAMIXType, therefore it belongs to a super/sub lattice (depending on the language) and it holds attributes, methods, structures.
FAMIX does not model explicitly interfaces and nested classes.
But a FAMIXClass can represent a Java interface (see isInterface query method). Java extractors can nest classes into classes and classes in namespaces.
For anonymous classes, classes are simply defined in methods.