PL, Private Line or CTCSS
Private Line is a registered trade mark of Motorola for a tone signalling technology where a code (frequency) is also sent while transmitting that cancels a blockage, which ensures that the transmitted signal can be heard by the receiver. Only those transmitters/receivers that are adjusted to the same PL code can communicate with each other. You cannot hear other users. This feature is also known as pilot tone, subaudio tone or tone squelch.