The class Triangulation_cell_base_3 is a model of the concept TriangulationCellBase_3, the base cell of a 3D-triangulation.
This class can be used directly or can serve as a base to derive other classes with some additional attributes (a color for example) tuned for a specific application.
The first template argument is the geometric traits class TriangulationTraits_3. It is actually not used by this class.
The second template argument is a combinatorial cell base class from which Triangulation_cell_base_3 derives. It has the default value Triangulation_ds_cell_base_3<>.
Note that this model does not store the circumcenter, but computes it every time the circumcenter function is called. See CGAL::Triangulation_cell_base_with_circumcenter_3 for a way to cache the circumcenter computation.