An acronym for Computer Aided Design, or Computer Aided Drawing, CAD is now how most architects and engineers prepare their drawings. As the entire industry and it's associated practices changed the role of CAD moved from not just being an internal drawing tool but as a general reference and framework for everyone involved on a project.
The architect's CAD files are passed to all the other consultants, including us, as a basis for the precise layout. Updates are sent out the same way.
From our perspective we often do what is effectively "tracing" over the top of the CAD files, however instead of tracing with lines we trace with entire building elements such as walls, windows and doors.