It looks like it's a macro key for sending a predefined ASCII sequence at the touch of a button. Apparently some paper teletypes had them as well, programmed by physically breaking tabs off a drum. (It looks like on early LSI terminals you could get your custom message burned on a PROM, later ones had nonvolatile memory.) In practice you'd use it to store a station identifier, or possibly an initialization/login sequence to send to the host as needed.