PL stands for Private Line. It is a sub audible tone that is built in to the transmit signal of a radio system. The radio that receives the transmissions is set up to listen for and receive that specific tone. During a transmission, if the tones match, the transmission passes through to the receiving radio. If they don't match, the transmission won't pass through.
The reason for PL is so that two agencies using the same frequency won't interfere with each other when transmitting on their radios on that frequency. The frequencies would have different PLs. Therefore, if they were both transmitting at the same time, they wouldn't step all over one another. Nor would they hear transmissions from the other agency on that frequency.
This applies to your scanner also. When you program the frequency for, say the PD for the town you live in, and also program the PL for that frequency, you will only hear them. You won't hear any other town that may be using the same frequency.
PL (Private Line) and CTCSS (Continuous Tone Coded Squelch System) are the same thing. DPL (Digital Private Line) and DCS(Digital Coded Squelch) are the same. DPL and DCS have nothing to do with the new digital radio systems. It is just another form of PL. It works basically the same way and serves the same purpose.
As far as viewing them on your scanner, they aren't automatically built in. You have to know them and program them in with each frequency. After you program them, you can go back and check to see what they
are. And of course, you have to have a scanner that is PL / DPL capable. Most of the older ones are not.
There are also some scanners that will search for the correct PL tone for a frequency. I have a BC 785D and if I put it in Tone Search mode, when it stops on a frequency I don't have a PL for, it will search and flash the correct tone when it finds it.
Hope I have explained this so you understand. Someone else may have something to add that I might have missed. But that's it in a nutshell.
