Sentence:  There are three principal feed bunk types for dairy and beef cattle : ( 1 ) Fence-line bunks - cattle eat from one side while feed is put in from the opposite side of the fence by self unloading wagons ; ( 2 ) Mechanized bunks - they sit within the feed lot , are filled by a mechanical conveyor above feeding <WSD>surface</WSD> ; ( 3 ) Special bunks - as discussed here , they permit cattle to eat from all sides . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary