david:
The perspective is a bit unclear (a section would show the horizontal relationships between elements better), but it looks like the lower part of the Curtain Wall is beyond the perimeter of the Slab. SEO will only cut the Curtain Wall that is vertically below the Slab, it is not an infinite cut plane. If this is the case, you need to add another Slab that covers the Curtain Wall for the SEO operator element, this Slab can then be placed on a hidden layer (if you delete the Slab, the SEO will be removed). If this is not the case, try posting other images that more clearly show the relationships between elements.
The lines running through the cut part might be there because you are showing the Scheme when editing the Curtain Wall.
Thanks for adding the signature.
David