In an agricultural setting a mower will cut grass and lay a 'swath', a row of grass which can then be worked for hay or silage, whereas a topper will cut and mulch. A topper is usually used to knock down weeds before they seed or lower the sward height, for example before putting sheep onto a new field to prevent um... how can i say it... Diarrhoea (great things spell checkers) we have a more vulgar term but it might not be deemed suitable in a public forum lol