Welcome Franz-Georg,
usually this depends on the used laser cutter. So the laser cutter shop has to tell you what is needed for a successful cut. Or, if you are the owner of the laser cuter, you have to study the documentation for DXF file requirements.
When you have specific requirements, we can help you how to achieve them with LibreCAD.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution