12v zener - T-Wrex build

[Schectadon]

The BOM calls for a 12v zener at D6. As luck may have it I don't have a 12V zener....only a 9v (1n4739), and 13v (1n4743a). I installed 25v + capacitors on my board so I could run the 18v if the urge struck me to reconfigure the jumpers. What is the Zener diodes purpose in this build? And if I socketed and plugged in the 13V zener what should I expect the result to be?

[Schectadon]

It looks to be in the boost section of the build.

[Schectadon]

Ok so I just placed an order to mouser for the diode and a few other items.