Sentence:  Hard <WSD>surface</WSD> cleaning in household application is represented by two classes of alkaline products : ( 1 ) the formulations made expressly for machine dishwashers , and ( 2 ) the general-purpose cleaners used for walls and woodwork . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary