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.
Your laser cutter company can provide you some specifications that are usually in the line of
1) Marks, for example you want to print some text on the material
2) Cut Through, areas you really want to cut out the material.
Some companies want you to put text vs cut's in different layers, other might want you to specify a color for each type of operation.
Once you have a good DXF the company with generate what is called a g-code file, this is something the laser cut company will have to do, because they know the best settings for they laser cutter. They can specify based on material the power that is needed, speeds and order of cuts and the size of the laser. For laser this can be important to prevent warping of the material.
Now what is (usually) VERY important is that all areas must be enclosed, so the g-code generator can properly generate the paths for the items to cut out. Usually the software will prevent warnings if the design is not correct. They should also be able to fix that easy by specifying a max distance to fill the gap.
They also usually cut out the outside of the project, not inside...
I would go back to them and ask them for the specification of what is important to them.
If it's a large project and it's the first time you work with them, do a test cut first on some scrap material. It will cost you some money, but can help learning the process and ensuring your deliver a good DXF and they can give you the correct cut.