The class Fixed_alpha_shape_vertex_base_3<Traits,Vb> is the default model for the concept FixedAlphaShapeVertex_3.
The class has two parameters : the traits class Traits which provides the type for the points or the weighted points. The second parameter Vb is a base class instantiated by default with CGAL::Triangulation_vertex_base_3<Traits>.
#include <CGAL/Fixed_alpha_shape_vertex_base_3.h>
Vb