As an intersection of 2 surfaces needs 2 surfaces, here the second part with an intersection of 2 surfaces :

One of my surfaces is defined as an extruded surface, the base extrusion being a curve in 3 parts : one line, one spline, one line.

Then the surface is splitted into 3 pieces.

If I don't pay attention to this fact, my intersection is also splitted into 3 parts.

How can I fix this fact when the intersection is built : where can I retrieve the 2 surfaces into my 3D sketch containing the intersection ?

The 3D spline is only defined as "the intersection of two faces0."

So, the fact that this spline isn't the  "intersection of two faces0", but the intersection of THE surface A, and THE surface B seems to be lost on the way !!!

My subsidiary question being : how can a guy having a  cartesian brain can work with this program ?