Motorola’s XPR5000-series mobile radios have two different output pins for a PA (Public Address) system. You would typically use this if you were using your radio’s microphone to speak over a loudspeaker to address, you know, the public. Their installation guide says pin 13 on the rear accessory connector is for “PUBLIC Address 1” and pin 15 is for “PUBLIC Address 2,” but the radio programming settings only describe “Internal” or “External” PA.
So here’s the difference:
Internal PA goes to pin 13/PA 1 and sends your voice out the speaker but does not send anything the radio receives, such as a transmission or alert. External PA goes to pin 15/PA 2 and also sends received transmissions and alerts. So I guess internal means inside the vehicle, where you already have a speaker, and external means outside the vehicle, where you likely won’t have a speaker and, should you be on the exterior, you will still hear calls.
Both audio signals will need to be amplified.