Most two-way radios have 38 CTCSS codes which stands for Continuous Tone Coded Squelch System. Many two-way radio manufacturers have different names for this feature. These are some of the manufacturers and their names: Motorola: "Private Line (PL)," GE/Ericsson: "Channel Guard", E.F. Johnson: "Call Guard", RCA: "Quiet Channel", and some simply call it "tone squelch," " privacy codes," or "interference eliminator codes". The most proper name is CTCSS.
These codes enable a two-way radio to keep the radio squelched (quiet) if the chosen code is not received with a transmission. In other words, the speaker of the intercom is not turned on even though a transmission is received. Technically speaking, a CTCSS tone code is a sub-audible tone/code which allows your radio to ignore (not hear) calls from other parties who are using the same channel. With CTCSS it may seem like you have your own private channel but other parties can still hear your calls if they set up their radio with the same tone code or no tone code at all.
Both receiving and transmitting radios must be set to the same interference eliminator/CTCSS code or it will appear that a radio is not receiving transmissions.
While CTCSS is said to transmit sub-audible tones (can't be heard on the radio's speaker), a radio is really continuously transmitting an audible tone below 300 Hz during a conversation. Each radio contains a filter to filter out anything below 300Hz so the tone is not heard even though it is there. Encoded tones are sent by the transmitter and decoded tones by the receiver to un-mute its speaker.
There is a digital version of CTCSS called Digital Coded Squelch (DCS).