Vod means output differential voltage which is equal to the difference between the two output terminals of a fully-differential opamp Vod=Vo(+)-Vo(-). The common mode voltage cancels out when you do the substraction. The plot Av vs. Vod probably just shows to you how the voltage gain of the amplifier starts dropping has your output stage starts to clip or to go deep into the linear region. I wouldn't think you would get much information out of a Av vs. Vcm plot because usually you want to select Vcm=Vsupply/2 so that you can obtain the maximum dynamic range at the output of your fully-differential opamp.