PL is a trademark name used by Motorola meaning "Private Line". There are other names used by different manufacturers and I can't remember all of them except for code guard. The words "tone squelch" are used widely in the communications industry to avoid use of trademark names. The name which describes the actual process is CTCSS or Continuous Tone Coded Subaudible Squelch.
CTCSS is fairly simple. A signal is added to the transmission at frequencies between 67 and 254 Hz or cycles per second. Sound at these very low frequencies is extremely difficult for most people to hear. Receivers have a filter which will not allow the processed signal to go to the audio phase of the radio unless it is accompanied by a tone of a given frequency. In the business service, frequencies are often shared by many businesses. The other businesses don't want to hear the traffic of the other users so a tone is selected by each business. This tone transmitted along with the signal of their radios and each radio is programmed with the same tone. It does not allow every users to transmit at the same time as that puts two or more FM signals transmitting on the same frequency at the same time. That is why a "monitor" button, which opens up the squelch to all signals is built into the radio. Mobile radios are often installed with a switch in the microphone which turns off the tone filter enabling the user to listen for any other transmissions when the mike is lifted. This feature does not open up the squelch completly (similar to turning the dial on your scanner unti you hear the rushing sound constantly) but turns off the filter so all transmissions, not matter what the tone frequency is, will be sent to the audio portion of the radio.
There seems to be an impression created by makers and sellers of family radio (FRS) that with a certain tone, full privacy is insured for the users. Many of these radios do not have a monitor button and since the user hears no other signals this impression is made stronger. Manufacturers claim 14 channels with 38 codes and then call them capable of using 532 voice circuits or some such rot. I was out on a hike one day and heard a couple of people talking on FRS radios while one was climbing a very signficant mountain. I had my scanner in the tone search mode and then programmed that into my GMRS (General Mobile Radio Service which requires a license which the majority of purchasers do not get) radio and called him up to get futher details on the conditions of the climb. I was planning to climb this peak soon and wanted to know how difficult a certain portion of the climb was where some snow and ice can make the trip a little difficult. From his reaction I could tell he was feeling like me talking to him was not possbile and might have been thinking someone just broke into a call he was making on a telephone.
On your PRO-96 you have several mode settings AM, FM, ED, MO, CT, and DC if I remember them correctly. The CT is CTCSS and the DC is CDCSS or Continuous Digital Code Subaudible Squelch or Digital Code Squelch. which instead of having 40-50 analog tones has dozens. The advantage of the digital tones is more accurate recognition of the tones and less false detection than analog tones causing the audio to be passed along from signals with different tones. These two different tone systems are only used on conventional systems, so don't worry about them on trunked systems.
If you program frequencies on the 96 with the CT or DC mode and don't know if a tone is being used, the radio will scan for any tones on the frequency. If you look at a lot of frequency lists, there are numbers like 103.5, 141.3, or 023, 413. The first two in my example are analog tones and the last two a digital tones. When you program in the tones, you can reduce interference from nearby strong tramsmissions on other frequencies. More an more communications systems are using tone squelch on their frequencies to reduce interference, which grows as the number of electronic devices, especially wireless ones, grows.
In some cases public safety frequencies are shared by many agencies, especially on county and city administrative systesm which provide communications for animal control, building inspectors, trash collection, waste water service, parking enforcement, etc. Each service will have a tone unique to that department. So if you don't want to hear trash pickup traffic and only want to listen to parking enforcement you can program the tone for that service and only hear them. If you can't find listings which include each services tone, just enter nothing after selecting the mode and the radio will automatically scan for tones on that frequency. See pages 42-45 of you owners manual and it is explained quite nicely. You then listen to the frequency a lot and note the content of the traffic for each tone displayed and eventually figure out which of them correspond to eeach service. If you know the tones but want to listen to all the services you can look at the display, note the tone being broadcast, and know which service is talking. For a particular system I listen to I have programmed a dozen different channels all with the same frequency but with different tones. I programmed each channel's alphanumerics with the label for that service so I have an instant display of what service is talking. I'm in a rural area where there aren't a lot of frequencies in use so eating up the memory in my scanners is not a major issue.
Another use of tones is to be able to select one repeater on a system with multiple conventional repeaters. During my career with the Forest Service on four different National Forests each Forest had multiple repeaters for its area, some of which overlapped in coverage. In order to select only one repeater for use at one time, we had an external tone box with a dial and each number on the dial represented a different tone and thus a different repeater. On some systems the tone transmitted on the repeater input frequency was also transmitted on its output. On some systems all the repeaters transmit the same tone on the output regardless of the tone used to select that repeater. If each repeater has a unique tone on the output, you can program several channels with the same frequency and different tones, as in my example in the last paragraph and be able to tell which repeater is being used when you look at the radio. Pretty slick I think considering my first scanner had eight channels and to change frequecies you had to take the radio case off, pull out a crystal with a pair of needle nose, and then insert another crystal which cost $5 and took 3 weeks to come in on a special order. You didn't do much listening for just curiosity on all sorts of frequencies.
Before CTCSS and CDCSS were developed, systems had different input frequencies for each repeater, all of which had the same output frequency. To select a particular repeater you selected the appropriate channel in your radio and all of them had the same receive frequency and with different transmit frequencies. That is a very inefficient use of the available frequenices. The first repeater system I used did not have subadible tones but audible tones in the 1000 - 2000 Hz range which were selected on an external box added to the radio. These tones were called burst tones because their length was fairly short each time a radio was keyed up. The disadvantage was not being able to distinquish the constant tones you were hearing all day, being used to select repeaters, from the tones used to make announcements and alert people to a dispatch. After a full regular shift or a long shift of 12-36 hours listening to all the beeping drove you crazy. Trying to get work done in an office where you had a base radio was diffucult and if you turned the radio down in order to stop all the beeping you might miss someone calling you or the ranger station.
My first National Forest didn't have any repeaters or remote bases except for the remote on a mountain near the town the dispatcher was in. If you were out of range of that remote base, you had to call a lookout tower and relay the voice traffic through them. Every message then, took at least twice as long to complete because it had to be repeated at least once, in the best of conditions, and more if someone in the chain did not repeat the message correctly. Some Forests were more advanced and you called the lookout to manually turn on the repeater located at the lookout. When you finished you told the lookout to turn it off. If the lookout wasn't staffed, as in the case of winter or very wet conditions during the summer, you were out of luck. I started my career in 1973 but the radio system was current technology for about 1950 or older, so I got to use some "old timer" equipment. We had "pack sets" which could have served as anchors for an aircraft carrier so when you went to a fire not reachable by vehicle, your shoulders got sore just carrying the radio. After we used every battery left in the world for these radios the system was replaced and we had repeaters. High tech!
OK, I hope you understand what PL is and how to use it on your PRO-96. I tend to tell people how to build a watch when they just asked the time so excuse me for the length of this explanation.
P.S. There are no stupid questions in my book, except for those that are not asked.
