These tones have many names:
Tones are often attached to a repeater's uplink frequency to control access or limit noise/interference. In commercial radio applications and invented by Motorola, the term is known as PL, which was short for Motorola Private Line. In the amateur radio community, the non-propriety term of Continuous Tone-Coded Squelch System, or CTCSS, if often used.
These tones were originally analog only (see chart below). Later, a digital method was also introduced known as Digital Coded Squelch, or DCS. These methods are only used on analog systems. Other methods for controlling access are available for different transmission modes and are discussed on their respective pages.
The tone system can also be applied to the downlink tone to limit interference and noise transmitted through the user's radio speaker. There are many sources of noise that can break the set squelch threshold, even though the repeater is not transmitting. Although the radio will still internally “hear” the noise, it will not open the squelch and transfer the audio to the speaker until the proper tone has been received.
Repeaters may not transmit the same tone that is used to receive (uplink). This is called “split-toning.” One should not make any assumptions as to whether or not a repeater is transmitting a tone and what the tone could.
Downlink tone is not a term that is recognized by most amateurs and you probably won't see it in many radio manuals. Although, Yaesu tends to use the term in reference to digital IDs. It's even been discussed on Reddit. To reduce confusion as to which tone is being referenced in terms of a paired frequency set (duplex with separate input and output frequencies, as repeaters, typically use, the downlink tone refers to the tone that the repeater is transmitting on the downlink frequency. If a user programs this feature into their radio, it is often known as “Tone Squelch.” It is not required that a user radio program this tone to receive the repeater's downlink frequency as the tone acts as a gate. When no tone is set, there is no gate.
RepeaterBook enters and displays data from the perspective of the remote (user) radio.
If a repeater has an uplink tone, you must enter it to access the repeater. But, if you don't program a downlink tone (often called a decode tone) into your radio, you can still hear the repeater uninhibited. In fact, many repeaters do not transmit a downlink tone. So why bother to program one in if you can hear the repeater without it?
The reason why is because you can hear everything else, too.
Especially in urban areas, there is a greater potential for RF noise. This elevated noise can produce signals with enough strength to deactivate the squelch on your radio and pass the unwanted signals. So, you elevate the squelch threshold, which works until you are in the fringe areas of the repeater and you need to lower the squelch threshold. Sometimes, the RF noise can come in full scale. These “birdies” can sound like analog paging towers and can be caused by mixing with the intermediate frequency of your receiver. These birdies do not transmit a CTCSS tone.
When you program a downlink tone, you ensure that the only signal that can break the squelch on your radio is a signal that is carrying that tone. It will also filter out distant repeaters on the same frequency, as long as they are not transmitting the same tone. You will have a quieter radio overall and won't be bothered by as many false signals.
Refer to the RepeaterBook database to see if the repeater is transmitting a downlink tone. If you set your radio to encode and decode tones when no downlink tone is being transmitted, you will not hear audio from the repeater at all.
All tones are in Hz.