questions about AD623

   I am doing my project with AD623, there are two questions I don't understand.
   1.On page 20 of datasheet of AD623, what's the '0.6V' stand for in the equations above Figure55? Why it adds 0.6V here? Does it mean the PNP junction voltage?

   2.'For a bipolar input voltage with a common-mode voltage that is roughly half way between the rails, VDIFFMAX is half the value that the previous equations yield because the REF pin is at midsupply.' Why here it says that 'the REF pin is at midsupply'?
    Thank you!