What does the +/- dial in the Lego Defender model do?
Based on a comment found in the overview on Brickset, and that all of the others levers and their function seem to be accounted for, it seem very likely that it is the transmission gear selector.  

-UPDATE BELOW-  

From Brickset:  


  "A new transmission system has two levers for engaging high or low
  ratios and a selector for changing gear – LEGO Technic’s most
  sophisticated gearbox yet."  


Source: https://brickset.com/article/45677/introducing-the-technic-land-rover-defender  

In other words, the transmission gears 1-?, rather than it's selection D/N/R and transfer case Hi/Lo.  The transmission looks too complicated to just be directional switch (D/N/R) and 1 gear range switch (Hi/Lo). 


  

I have no idea how many gears it has, or if I am even correct.  It's all just speculation based on my observations.  Either way the set looks amazing!  I'm a sucker for vintage cars and all things 4x4.  

Just confirmed from Lego.com:  


  Opening doors allow access to an elaborate cabin with a working
  steering wheel, detailed dashboard and a new-for-October-2019
  transmission system with 2 levers for engaging high or low gear ratios
  and a selector for changing gear – the most sophisticated LEGO Technic
  gearbox to date! The interior also features forward-folding rear seats
  that give visual access to the 4-speed sequential gearbox.
  https://www.lego.com/en-us/product/land-rover-defender-42110  


So it's a 4-speed transmission, nice!