This drawing has layers for panels and equipment, conduit and wiring, underground conduit, miscellaneous items, and notes. This drawing contains all the electrical details. Many of the layers mentioned are for setup and reference purposes only. For example, once the sheet borders and title block are established the designer doesn't need to work on those layers anymore. Other layers, like the background layer are primarily reference layers, and are never altered by the designer.
Some layers serve double duty as both a reference layer and an active work layer.
For example, the mechanical designer actively works on the HVAC equipment layer to locate compressors, pumps, and air handlers. When complete, that same layer becomes a reference layer for the electrical designer laying out conduit and wiring. Reference layers are very important to have so that system layout and interference checking are easy to perform. All CAD systems allow the designer to make changes in only one layer at a time, but the designer can specify any combination of layers to be visible as work is performed. For example, if a designer is working on the ductwork layer, he will probably have the background and HVAC equipment layers visible at the same time.