The black lines represent unit boundaries. The solid black line at the edge of the map is the boundary line between 7th Army and Panzer Group West (which is what I use to define the crops for my maps). The shorter black lines with the hash marks are boundaries between subordinate corps. The maps don’t show divisional sectors. In some areas of Normandy the German divisions have been so chopped up and intermingled boundary lines would be meaningless anyway.
Oops, my bad! Upon further review there are divisional boundaries. They are the short lines with the bar on the end.