This note in datasheet on input Common Mode Range -
VSSA to 6 × VBG range is used for direct measurement of VDDA power supply. Actual scale is limited to VDDA.
This makes sense, that input range, in a CMOS device designed for R-R performance, is limited to
supply rails +, if we are lucky, a diode drop.
I agree with Daana, the Vdda limit is around 5.3V.
Hence we cannot have have inputs beyond this to the adc.
The reason why we have an option of choosing up to 6 V Range is to improve the linearity of the ADC while operating in the 0 to VDDA range. Higher the range,improved is the linearity.!