The following occurs in NX$.$ A block generated via sketch and extrude $($pic $1),$ next a free form

curve $($three degree spline$)$ is drawn in a sketch on a plane outside the model $($pic $2).$ After

that a sheet is generated, based on the spline $($pic $3).$ Next the plane is used to trim the body.

$($pic $4).$ The question is now:

Why can't you analyse the surface with the normal $"$B$-$surface Information", i$.$e$.$ why is the

surface not considered a free form surface?

Pic$1$

Pic $3$

Pic $2$

Pic $4$