In the essay’s account of Enzo Mari, exact form is the shape a designer can justify through an object’s purpose, material and method of manufacture. Its test is whether each choice has a reason.
Illustrative example
Consider a stackable tray cut from a single sheet of metal. Its folded rim stiffens the edge and leaves room for a hand to lift it. Both the fold and its depth follow from use and fabrication; neither is there merely to suggest a style.