Surface converted into a curve ... feature or bug?

Why is this surface converted to a curve by the “Curve” component ? Because it is periodic ?
Feature or bug ?

Surface converted to (14.4 KB)

Did I manage to stump the whole community with that one ?

I guess because it can join all the naked edges in a single curve.

Well, OK, but it shits in the boots of my logic here.
I need to really sort curves.

If an automatic operation does not work for you, replace it. That’s not the designed way to extract the curves from a surface, it’s just a shortcut.

It’s the other way around : I don’t want to extract a curve from a surface, I want to filter actual curves.
I guess I’ll use Elefront’s “Filter by type” component :

In any case, there should be a clear description of the data conversions operated by the “container” components :

Ahh ya, GH1 does not have any type support, I also faced this problem sometime.