How to use layer and why we using that one. And what is geometrical set we using that one too...
As per my knowledge DS doesn't support layers in either Catia V5 or Solidworks. Creo has layers option because Creo doesn't support multi bodies but catia does. Their argument is they have bodies and geometric options for organization and so there's no necessity of layers.
Let me know if you find out anything that states DS supports layers. I would like to know if they do.
the tree is the layer in catia
CATIA V5 does indeed have layers; by default they are numbered 0 thru 255 but they can be renamed. Drawing and 3D elements can be assigned a layer within the Properties panel.