small erosion gullies designated by a single line

I don't understand the statement "small erosion gullies (1-2.5 meters in width and less), which are designated by a single line". In JOSM lines can be drawn with a distance of 0.001 meters.

--sperlingskauz 22:04, 23 July 2017 (UTC)

Same way as a number of trees can be represented by a natural=wood polygon or a natural=tree_row line, in the same way large gullies is convenient to draw with a polygon (not closed), but small ones with a single line.