References
A reference attachment is a model attached to and displayed with the active model for plotting or construction purposes. You can attach as a reference a model that resides in the open DGN file, another DGN file, or a DWG file.
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.