References (Reference files) are numerous DGN files and/or raster (image) files that are combined and attached to a single DGN file. The contents of these files can be viewed as a background in the active open DGN file. Reference file content can be moved, scaled, rotated, mirrored, clipped and masked to manipulate the information as an attachment to the active open DGN file.

Since references appear as background information, you can easily draw over the contents without affecting the original files. Tracing over a reference or raster file to build information for a model is often accomplished when creating 3D site models for example.