Autodesk's lightweight, compressed file format for sharing and marking up 2D and 3D design data without the original authoring software. It's Autodesk's answer to a universal, view-and-markup-only export - much smaller than a native RVT or DWG, viewable in free tools, and unable to be edited back into the source model. Useful for review packages sent to people who don't need (or shouldn't have) editing access.