"Prohibitive cartography has its own graphic rhetoric. Because efficient enforcement depends on well-defined territorial restrictions, the primary symbol on most prohibitive maps is the boundary line, underscored perhaps by labels and contrasting colors.
By convention, small-scale political maps printed in colors rely heavily on dissimilar hues—when France is green and Germany is purple, there’s less need for the prominent dot-and-dashed “international boundary” symbol common on graytone maps.
The mapmaker can emphasize national sovereignty with thick, solid black lines—the most prominent symbol on many State Department maps—or underscore disputed or otherwise tentative boundaries with equally thick dashed lines. By contrast, the thinner, less prominent dot-and-dashed line is a convenient, readily understood code useful with less strident atlas maps, on which political boundaries have a weaker claim to continuity than roads, railways, and rivers.
By chance, line symbols with periodic gaps afford a more accurate representation of boundaries like the U.S.-Mexico border, which is far more permeable than an unbroken line might suggest."
MONMONIER, Mark. “No Dig, No Fly, No Go. How maps restrict and control”. The University of Chicago Press. Chicago, 2010. p.3
Via rhetoric in graphic design