The previous section was about UNIX commands you enter at a shell prompt. Some UNIX commands have commands of their own. (For examples, look at the more, mail, and pg commands in Chapter 3. Text editors like vi and emacs also have their own commands.) Once you start the command, it prints its own prompt and understands its own set of commands (not UNIX commands).
For instance, if you enter mail, you’ll see a new prompt from the mail program. You’ll enter mail commands to handle mail messages. When you enter the special command (q) to quit the mail program, mail will stop prompting you. Then you’ll get another shell prompt; you can enter UNIX commands again.