Simply, the ADC has a number of counts, for example lets say 255. The reference voltage represents the count of 255. If your voltage divider supplies a voltage of half the reference voltage, you will get a count of 127 or so. If that voltage changes to 3/4's of vref you should get a count of 191 counts, and so on. So it all comes down to the voltage dividers and the voltage supplied to them which will get you a fraction of the reference voltage.
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