It basically represents the fitness of dimensions of a member. Iit means the arrangement of material according to the expected role of a member. for example if a member is to act like a beam then its material should be placed to maximize its height rather than to enhance its width. because height will increase its inertia and it will support more load.
in a circular member to support torsion, the material near the center is not fully utilized so a hollow member is used because with the same material it will have more inertia that a solid member.