Working with design models, you can create one or more discrete models, simply, within a single DGN file. Using the Models dialog , you can quickly switch between the various models in the DGN file. Where a DGN file contains many models, you can set a filter so that only particular models display in the dialog. You can turn the filter on or off, as required, but filters are not saved from one design session to the next.
Similarly, you can create sheet models to compose your design drawings. The drawings typically consist of references of the design models. References can be from the open DGN file, or from other DGN files on disk. As with design models, you can use the Models dialog to switch between the sheet models. At any given time, only one "active" model can be displayed, either a sheet model, a design model or a drawing model.
A drawing model is the intermediate between the 3D design model and a printable sheet model. It is an unbounded white space that contains one or more views of the design model. You can use a drawing model for centralizing annotations that need to be shared across multiple sheets. If you want to extend a saved view by adding annotations, dimensions, callouts, and other embellishments, create a drawing model for it. When a drawing model is attached to a sheet, the drawing model's annotation scale is used as the attachment's default detail scale.
When you first create a DGN file, it has a default design model ready to place elements. When you open the Models dialog , the default model has the name Default, with the Description as Master Model. You can use this model and, if necessary, change its name and description to more appropriate values.