One of the basic principles of a good model design is to divide the model into a series of layers, each layer having a specific function within the model and a related set of modeling activities.
Originally IBM Cognos recommended the use of two layers, a data layer and a modeling layer. This was later refined by the inclusion of a Presentation layer, and a separate Dimensional layer if the Dimensional Modeled Relational (DMR) data is also used in the model.
The layers are built upon one another, with the lowest level being the data layer, the modeling layer as the middle layer, and the top layer being the presentation layer. The dimensional layer can be used to replace the presentation layer, or can be used in addition to the presentation layer.