outer integration = 1 to -1 (limits of int.)

inner integration = sqrt(1-x^2) to -sqrt(1-x^2)

function = x^2 + y^2

the part i dont get is the question states to sketch or describe the region, im good with that, but i dont understand what is meant by the following:

What region in the plane comprises the lower boundary of this region?

Can anyone help?