CCMD is used for several programs on our UNIX systems, notably the mail manager mm.
Probably the most important feature is instant help. At almost any point when a program written with CCMD is waiting for input from you, you can type a "?", and it will tell you what it is looking for. One exception is when a program is parsing a paragraph, at which time the question mark will simply get inserted into the text like any other character. If you have partially typed a command, CCMD will only show you things that match the current input.
Another feature is command completion or recognition. This means that when a CCMD program is parsing, you can hit the escape key (usually labeled "ESC", but sometimes labeled "ALT" or "META") or TAB key (sometimes labeled "->") and the command will be completed if it is unambiguous. Control-F works similarly to tab and escape, but completes a little less. If what you have typed is ambiguous, CCMD will beep. You can then hit "?" to find out the available completions. For example, if you were typing in the file name "foo.bar" and there were no other files in your directory beginning with "f", you could simply type "f<ESC>" and the program would echo "foo.bar ". If you typed "f<Control-F>" CCMD would echo "foo" and wait for you to finish it. This is useful when specifying the name of a new file.
A third feature is command history. Using control-P and control-N you can redo previous commands. Control-P goes backwards through the list of your previous commands, while control-N goes forwards towards the more recent commands. You can edit these commands just as you would a freshly-typed one.
Related to command history is command retry. This means that after you have typed a command and hit return, then realize that was not what you meant to do, you can redo the command. When you get the prompt back from the program, if the very next character you type is control-H, MM will redisplay your last command. You can then delete the offending parts and run the command you wanted. This saves you from having to retype the entire command.
CCMD will accept comments on any line -- this is most useful inside files run by the "take" command available within most CCMD programs. The comment start string is typically "#", and the comment end string is end of line.
In case you should want to input any of these special characters, CCMD also provides a quoting character, control-V. Control-V followed by any character simply inserts that character into the current command.