What is the function of a transceiver’s PTT input?

PTT stands for Push-To-Talk. It is the primary control line or switch, most commonly found as a button on a microphone, that tells the transceiver to stop receiving and begin transmitting. In most modern amateur radio transceivers, this PTT circuit is activated by connecting the PTT pin to the chassis ground. Grounding the connection completes the control circuit and triggers the radio's transmit relay. The distractors are incorrect because continuous wave (CW) keys use a completely separate keying input, and tuning tones are generated by internal oscillators rather than the PTT switching circuit.