What is the 'CB2 Sound'?

THE "CB2 SOUND"
     
        This is the sound made by sending square out of the 6522 IC on some
        Commodore computers.  It is called 'CB2', since that is the name of
        the pin on the 6522 that outputs the waveform.  I won't go into a
        complete description, except to say that most models of the PET
        had the capability, and most PET owners used it as the ONLY sound
        source, since the PETs did not have a sound chip.  Although the VIC
        did have some sound capabilities, by that time Commodore had 
        realized its widespread use and included some information on it in
        the Commodore VIC-20 Programmer's Reference Guide.  For more info,
        reach for your nearest VIC PRG and look at page 232.