What term describes the use of a sub-audible tone transmitted along with normal voice audio to open the squelch of a receiver?

CTCSS stands for Continuous Tone-Coded Squelch System. It is a continuous, sub-audible tone (usually below the typical human hearing range for voice communications) that is added to your transmission. It acts like a specialized "key" to open the squelch of the receiving station or repeater, preventing them from hearing static or interference when you are not transmitting. The other options serve different purposes. Carrier squelch relies solely on the raw strength of the incoming RF signal to open the audio path, not a specific tone. Tone burst is a short, audible tone used to activate some older European repeaters, while DTMF refers to the audible "touch-tone" keypad sounds used to send remote commands.