Sorting issue after Surface Split

Hi all,

I have checked similar topics but I couldn’t figure out.

I couldn’t correctly sort the geometries after I used Surface Split.
I would like to extrude only the wanted part (B part, Image) but it is only working when the division of the initial surfaces are less than certain number.
I do not know how to get rid of the red color surfaces in the A part.

Surface split.3dm (124.4 KB) Surface (16.3 KB)

Could anyone give me advice?
Thanks for your advice in advance.

In this case, Region Intersectionis not that tough and expansive solution.

Surface (11.4 KB)

Hi Kim,
Thanks. it works well.

Hi Kim,

I can’t use the solution in the other shape.

Surface split-1.3dm (38.9 KB) Surface (12.7 KB)

What makes this difference?
Do you have any thought?

Surface (14.9 KB)

1 Like

Thanks a lot.
So I should define plane too if Region intersection is not working properly.