Layers partition a map into one or more overlays. Layers are like transparencies in that you can see all layers at the same time. You can isolate different objects by placing them on separate layers. Only one layer can be active at a time so commands can only be applied to the active layer. Objects on other layers remain unaffected by any changes you make on the active layer, except those commands that affect the map coordinate system, the map projection, or the map scale.
Each layer can contain only one thematic map, but you can place different thematic map types on different layers within a single map window. To create a drawing with several thematic maps, you must use a new layer for each thematic map.