The plot legend shows Abs[f[z]] in grayscale and Arg[f[z]] in hue colors. This depends on the ColorFunction. The tick value (12 or 7.2) indicates where White starts. The cutoff is probably chosen by ComplexPlot so that (1) no more than a small fraction of the plot is white and (2) the rest of the plot has a large "dynamic range" of color intensities.
You can get both an upper and a lower tick, showing the cutoffs for white and dark gray, respectively. Example from the documentation for ComplexPlot, option ColorFunction, with PlotLegends added:
ComplexPlot[(z^2 + 1)/(z^2 - 1), {z, -2 - 2 I, 2 + 2 I},
ColorFunction -> "QuantileAbs", PlotLegends -> Automatic]
