Sometimes, we have to define which faces are in front of anothers in a same mesh.
We have several tools that help you to select and mark the faces. This function are very usefull for courtains, bed, etc.
Many softwares force you to use several meshes to represent one object,and then they have problem to join the meshes or the result is not correct.
We have many functions that allow you to generate only one mesh for one object, even when this object has a difficult surface.