You are always drawing on a layer. It may be the default layer or a layer you create and name yourself. Each layer has an associated color and linetype. For example, you can create a layer on which you draw only centerlines and assign the color blue and the linetype CENTER to that layer. Then, whenever you want to draw centerlines you can switch to that layer and start drawing.
All layers and linetypes are kept within their parent Collection objects. Layers are kept within the Layers collection, and linetypes are kept within the Linetypes collection.
For more information about working with layers, see â€śWork with Layersâ€ť in the User's Guide.