I am totally bewildered by a radio feature and can't find an answer anywhere.  I found one single page that says it reduces background noise when there is no audio is present, but that absolutely isn't happening on any of my radios.  Hopefully some of the smart people here know what this stuff is.   I have several Yaesu radios that have a squelch type called "PR" in the radio menu, and "No-Communication Squelch" in the manual.  There is nothing in any of the manuals I have that explain