The layers of a document are analogous to transparency overlays. Architects and engineers can use the layers function to separate different kinds of information. One layer can be the basic floor plan, another the electrical wiring, and a third the plumbing details. The layers function allow you to keep the underlying format for use time after time and change upper layers as needed. The number of layers in a given document are only limited by the memory of the machine you are using.
There is only one active layer at a time. If you draw an object, it will appear on the active (or 'current') layer. You can select objects that are not on the active layer by holding down the option key while you click on the object. You can use modifiers or the selection filter to assist in selecting objects across layers.
You cannot group an object on one layer with an object on another layer.
If you copy an object you will lose the layer information for that object. In other words, when you copy an object from layer 2 in one file, it will paste to the current layer of the document you paste it into, whether that is layer 2 or not.
The maximum number of layers is determined by the amount of RAM in the Macintosh. According to our past experience, you will most likely not run into this limit.. You can name the layers, as well as hide, show, delete and create layers in the layers floating window. You access the window by selecting the Show layers command under the Layout menu (or shft-cmd-l).