I use the designer for making SVG files for laser cutters. Laser cutters have an internal logic to determine if a line is a cut line or an engraving line or if it's a 2D area for area engraving. Area engraving is a bit different, because the laser head is not like "follow the line", but more then "print like an ink jet printer". Obviosly cutting is not possible without a "cut line"
The internal logic is determined by the thikness of the line. In other programs like Inkscape or Corel (as in this screenshot) you can choose "Haarlinie" (german for hairline) as thickness. Then and only then, a laser cutter will recognize this as a cutline.
Maybe you can add the thickness "hairline" in Designer too?