Layers are the way information in a cad drawing are organized. We will use your layer conventions or  default to AIA standards. AIA standard layers use the first letter to tell you what the layer is about, for example if we use the layer A-Wall the "A" stands for architectural. M would stand for Mechanical and so on. Layers also have colors and line types assigned to them.

The next part is -Wall, which means the walls will be put on this layer.  

We will use a simple layer convention if none is supplied by you.

We will also use our own default .ctb file unless you supply us one. The ctb file tells how each color should be plotted. Each  color is  assigned a different line weight and can be "screened" so they will show up as light images, useful when doing such drawings as electrical and mechanical drawings over base plans.