The global force generalizes the concept of Center of Pressure. See for instance Center of pressure

Since there doesn't seem to exist a universal definition of the Center of Pressure in 3d, it is calculated in flow5 as:

CP.x = Sum(Fn_i.x_i)/Fn

CP.y = Sum(Fn_i.y_i)/Fn

CP.z = Sum(Fn_i.z_i)/Fn

where Fn is the component of the resulting force along the wind normal, and the sum is carried out over the mesh panels.