Same mask and shape as in my previous post but using the Combine Shapes > Subtract shapes.

Result: The non-masked area is subtracted from the bitmap.

Same Mask and shape as in my previous post but using the Combine Shapes > Interset Shapes.

Result: The area not masked is the result, similar to Duplicate and clone except the original shape and mask are no longer in the work area.

Essentially the shape drawn on the mask is used exactly as a shape drawn in the work area and used as if it were selected along with the original bitmap. The mask does not require both objects are selected.