When working on Revit projects, you can import/link DWG, DXF, DGN, RVT and image files. These files can contain 2D and 3D geometry to help in the construction of your projects. Raster images can be imported as background images into your Autodesk Revit project or as visual aids needed during the creation of a model.
When you've exported the project to a DWG file, RVT links are comparable to xrefs in an AutoCAD drawing.
For more information about the Import/Link features of Revit, refer to the Autodesk Revit Help file.