Region curve/surface question

I'm having trouble constraining cut to region.
I select my object. Choose tool. Create box. then...

Selected a closed region curve which I have created around the outer margin of my object. I assumed that selecting this curve would constrain my subsequent surfacing operations to within the region curve.
However each time I run simulation it cuts the entire block including outside the region curve.
I tried raising the curve height to top Z. Then at lowest Z. No joy.
I tried extruding a surface from the curve and adding a clipping plane at lowest Z. No joy.

Interestingly using the curve alone actually worked once, but I cannot replicate that result.

Any ideas?

disregard! It was a bad curve.