The PL tone is totally up to the local system's admin on what one(s) they pick for their users (or even if they descide to use them). In most cases for scanner use you don't need to worry about it since they only restrict what your receiver will recognize. If you have two agencies in your area that share a frequency (say you're in a rural area between two cities) you may want to program in the PL tone so you can get the one city's PD channel and not be bothered by the other city's trash pickup that happens to use the same frequency.
If you have your scanner set to pick up 156.00 MHz with a PL tone of 102.5 than you'll only get signals on 156.00 that have a tone of 114.8. If they set up a SWAT operation next door using a special PL tone for SWAT operations on the same frequency (say 156.00 with a tone of 97.4 you will miss the SWAT transmissions, but still get the routine calls to dispatch for license checks that use the standard 156.00/PL 114.8. Now if you have your scanner set up to not use any tones (carrier squelch), you'll get both operations without any issues. While it's quite rare that SWAT will use the same frequency as dispatch, the point remains valid.
With many scanners, you can program the frequency in and when a signal is received have it find the PL tone being sent with that signal for you. Check your scanner's manual for specifics on how to do this with your make & model (they differ greatly at times).