calculation methods for curve

[[Ba...]]

Have anyone know about the calculation method of deviations for the curve. 3D curve has several methods for calculate the deviations. The cookbook explaned following methods. but I am not sure I have actually understand it.
– ➤ Nominal Vector Direction [⇨ 1-67]
– ➤ Act -> Nom [⇨ 1-67]
– ➤ Nominal in plane [⇨ 1-68] (only for 3D curves)
– ➤ Actual in plane [⇨ 1-68] (only for 3D curves)
– ➤ in X direction [⇨ 1-68] (only for 3D curves)
– ➤ in Y direction [⇨ 1-68] (only for 3D curves)
– ➤ in Z direction [⇨ 1-68] (only for 3D curves)

here, i am interested in the "Nominal in plane" method.
for the picture above, all points of this curve are at the section Z=3.644, but the Z component of vector are not 0.
The default calculate method is Nominal vector direction, after i sacn it, the actual points is not actully at the Z=3.644 but more or less a bit, the point 1 is 3.644079,other points are 3.6441 or 3.6439 etc... .
I think because the surface is a bit minus material, so the probe go acrossed the nominal point, so the Z value of the actual point is a bit bigger than 3.644, showing in the picture below.
So, how can I get the point actually on the Z=3.644?, at "point" feature, it can get by the "net point" space point mode.
I think it can not be probe actually.
But it maybe get by mathmatic calculation, I found the "Nominal in plane" calculate method, when I toggled to this calculate method, the Z value of all the actual points are changed to 3.644. it seems realized what I want, but I am not sure.
below is the explaination about the nominal in plane i found on cookbook .
but what is the intersection plane? i never defined it. and how the actual point project onto the intersection plane?
Thanks in advance.

[[An...]]

You want this point,and you get this point.
All actual points have Z=3.644.
The actuals in X and Y are variable.
The plane is defined by the nominal Z-value.

[[He...]]

The intersection plane is the plane that connects all the points of the curve. I.e. if you create your curve from a plane-section that plan is the intersection plane.

If you create a 3d curve where not all nominal points share a plane you will not be allowed to use the nominal in plane option.