HIDKeys: usage of PD1

Hi all.

The HIDKeys sample application provides for 17 keys.
The pin assignments include "PD1 debug tx".
The circuit diagram shows PD1 disconnected and yet a portion of the "hardwareInit" function states...

   
DDRD = 0x07;    /* 0000 0111 bin: all pins input except USB (-> USB reset) */
   j = 0;
   while(--j){     /* USB Reset by device only required on Watchdog Reset */
      i = 0;
      while(--i); /* delay >10ms for USB reset */
   }
    DDRD = 0x02;    /* 0000 0010 bin: remove USB reset condition */

What is the reason for this assignment and treatment of PD1?

Could PD1 be used to expand the 17 keys to 18 keys instead?

Many thanks,

Ross