AutoCAD uses hundreds and hundreds of system variables to control and report the state of the software and its drawings. CmdDia is one that's come and gone over the years, and now its back with a new meaning:
CmdDia: Toggles display of QLEADER’s inplace text editor.
In earlier releases, this system variable toggled the PLOT command between dialog box and command-line interface. In AutoCAD 2000, CmdDia's function was replaced by PlQuiet. As well, you can use PLOT (dialog boxes) or -PLOT (command line prompts).
Thanks to Bill Fane for pointing out the resurrection of CmdDia.