Depending on which Cgal kernel is used, different versions of this global function are available. This is described below.
With the basic 3D Kernel (see Chapter 9)

 
Compares the Cartesian coordinates of points p and q lexicographically in xyz order: first xcoordinates are compared, if they are equal, ycoordinates are compared, and if both x and y coordinate are equal, zcoordinates are compared. 
With the 3D Spherical Kernel (see Chapter 12)
#include <CGAL/global_functions_spherical_kernel_3.h>
If this kernel is used, in addition to the function and the combination of 2D types described above, another version of the function is provided.

 
Compares the Cartesian coordinates of points p and q lexicographically.  

 
Compares the Cartesian coordinates of points p and q lexicographically. 