When you draw or place elements in a DGN file, you are creating a model. A model is a container for elements. It can be either 2D or 3D, and is stored as a discrete object within the DGN file. It may be helpful to think of a DGN file as a stack of cards, with each card being a model.
When you first create a new DGN file from one of the seed files, this provides the empty container setup with a default model ready for you to create your design. If you use a 2D seed file, then the default setup is 2D, while a 3D seed file defaults to a 3D setup. In either case, you can create both 2D and 3D models in the open DGN file.
Every model has its own set of eight views. The model whose views are displayed or available for display at a given time is the active model.
Using the Models dialog , you can switch quickly between models in a DGN file.
Alternatively, you can use the View Groups window to rapidly switch between models visited in the current design session. These can include models in other DGN files.