Simcenter STAR-CCM+ Pressure coefficient in horizontal, CpX and vertical, CpZ direction, respectively;

Wow, what is the signification of that title, a pressure coefficient is a scalar value and defined in STAR-CCM+ under:

Physics Simulation > Fluid Flow > Flow Reference > Flow Models Reference 

Now, computing Cp with respect to direction, why and how is this been done?

Why is it been done?

Those coefficients help to understand where drag and lift forces are actually generated. You can imagine them as a 

non-dimensional distribution of pressure forces. Looking only at the pressure on the surface doesn’t tell the complete

picture about the direction of the pressure force. Indeed, having any pressure value on a horizontal surface will not

contribute to the drag of a vehicle. On the other side, you can have any pressure value on a vertical wall, which will

make zero contribution to the lift.

How is it been done? 

What we need to consider here, is the normal component of the Area vector times the pressure coefficient, Cp, s.t.:

$CpX = $Cp * $$Normal[0]
$CpZ = $Cp * $$Normal[2]

If the surface is horizontal, $$Normal[0] = 0, hence CpX=0 no matter the value of pressure acting on the surface.
This explains why the CpX picture is all grey apart from those surfaces which have a relevant horizontal component

of the normal (e.g. helmet, tyres,..).

Of course, when Cp=0 any CpX or CpZ are zero as well, irrespective of the surface orientation.