Covered means a huge mass over the whole corner, such as a 15 ft high mound or a stone fill 35 to 40 ft long. A 2 ft high chabutra in the South-West, the walls of a small house, wardrobes or a small landscaped bed are not covering; the trouble starts when the whole zone, for example the entire South-East area, is buried. The rule applies to the corners of the total land at the boundary wall, not to covered areas inside an industry.
Case: on a tilted industrial plot, a boulder fill about 35 to 40 ft long and 22 ft wide was put in the West corner on another consultant's advice. It suppressed the zone of Saturn, and the industry ran at a loss until it neared closure. Vedic texts mention the covering of corners as a defect.