What is TTY? How it is used?

Am using iPhone 3GS I went through phone settings I found TTY in phone option can you please tell me what it is and how it is useful

A TTY is a special device that lets people who are deaf, hard of hearing, or speech-impaired use the telephone to communicate, by allowing them to type messages back and forth to one another instead of talking and listening. A TTY is required at both ends of the conversation in order to communicate.

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