A reference attachment is a model attached to and displayed with the active model for plotting or construction purposes. A reference cannot be modified unless it has been activated. You can attach as a reference a model that resides in the open DGN file, another DGN file, a DWG file, or any other file type that is available in the Attach Reference dialog.
Elements in a reference display as though they were in the active model. Although you cannot manipulate the elements in a reference, you can snap to them and even copy them into the active model.
Common Usages for References
The most common usage of references is in the creation of design compositions. Design compositions are used by engineers and other technical professionals to communicate through the visual content of their designs.
To create a design composition, you build a design model consisting of a working collection of references used in the performance of particular engineering tasks. For example, you may attach as references a collection of survey points as a guide for placement of additional geometry. Reference attachments in a design composition are usually coincident.
Preferences Affecting References
User preferences that affect reference operations can be set in the Preferences dialog Reference category.