Stereo phone jack: purpose of terminals 1 and 6
It's a socket for a 4-pole jack as used on mobile phones. On the PJ-W27B2-06D (leftmost version) you have


Tip: speaker left.  Pin 2.
Ring 1: speaker right. Pin 3.
Ring 2: Mic or ground Pin 6.
Sleeve: ground or Mic. Pin 1.


Pins 4 and 5 allow audio through to internal speaker, for example, if the jack is unplugged.

If you don't need the mic line then connect 6 to 1 to give better sleeve connection.