From this news group posting by Mark Zbikowski himself:
The differences between .CMD and .BAT as far as CMD.EXE is concerned
are: With extensions enabled, PATH/APPEND/PROMPT/SET/ASSOC in .CMD
files will set ERRORLEVEL regardless of error. .BAT sets ERRORLEVEL
only on errors.
In other words, if ERRORLEVEL is set to non-0 and then you run one of those commands, the resulting ERRORLEVEL will be:
- left alone at its non-0 value in a .bat file
- reset to 0 in a .cmd file.