CTCSS
Continuous Tone Controlled Squelch System. Continuously superimposes any one of about 50 low-pitch audio tones onto the transmitted radio signal.
CTCSS is often called 'a PL tone' (for Private Line, a trademark of Motorola), or simply 'Tone Squelch'. A common feature of many radio communication schemes, CML's CTCSS products offer a wide range of tone-set 'tones'.