The 14-bit ADC hardware allows absolute and relative modes of measurement.
As Figure 6 shows, relative measurements use resistances (sensors) that are independent of the supply voltage. This is the typical way to use the ADC. The advantage is independence from the supply voltage; it does not matter if the battery is new (Vcc = 3.6 V) or if it has reached the end of life (Vcc = 2.5 V).
As Figure 7 shows, absolute measurements measure voltages and currents. The reference used for the conversion is the voltage applied to the SVcc terminal, regardless of whether an external reference is used or if SVcc is connected to AVcc internally. An external reference is necessary if the supply voltage AVcc (the normal reference) cannot be used for reference purposes, for example a battery supply.