This is when a communications program will send information (either that you type or from a file) to your screen, as well as to the other modem.

A modem feature that allows it to send out copies of commands.

Specifies that the characters you type are to be "echoed", or set to your terminal for display, by your terminal. This is needed only when neither the remote computer you are using nor the communications system echoes the characters, as in half duplex communication.