The class Triangulation_face_base_2<Traits,Fb> is a model for the concept TriangulationFaceBase_2. It is the default face base class for basic and Delaunay triangulation.
These default base class can be used directly or can serve as a base to derive other base classes with some additional attribute (a color for example) tuned for specific applications.
The second template parameter has to be a model of the concept TriangulationDSFaceBase_2 and will serve as a base class for Triangulation_face_base_2<Traits,Fb> . CGAL provides a default instantiation for this parameter which is Triangulation_ds_face_base_2<>.