IMHO combining shapes and parametric selections does not work well and is at best confusing, and possibly buggy.
I can see how you would specify how a shape lower down the list interacts with the shape above it, but I don’t understand what the first shape interacts with, and the manual is not clear on this, to me.
I think this part of dt would benefit from an overhaul, I made a suggestion here Ideas on how dt’s Mask interface could be improved
some time back.