When a rectangular region is defined using an appropriate style, which value matches the specified edge of the clipping region to the edge of the parent element?

Auto function.

When clipping anything using an editing software, the auto function takes control of the clipping region and the edge of the parent element. This is pre-programmed into most image editing software to give users an easier time with selection.

There is also an option for other types of clipping like setting the dimensions of the clipping, setting the margin of the clipping, and even selecting only the desired areas with a free tracing tool.