Oilcanning refers to ripples which can appear in the surface of the panels. Most common causes are inherent and unavoidable stresses in the shape of the metal, and imperfections in the flatness of the surface being installed on. Other causes can be improper panel formation and improper installation.

Requiring this disclaimer is not uncommon.

You may wish to ask the contractor if they have any options for ribs or striations in the panel which can help to mask the appearance of oilcanning.