Precision resistor on ADC voltage reference pins
The designator of the component should be a clue-\$R_{REF}\$.. it's the REFERENCE resistor to which your RTD value is compared. 

The calculation of the RTD should use the ratio of the ADC readings of the sensor and thee reference resistor. 

You MUST have an accurate and stable reference resistor if you want an accurate and stable reading. On the other hand, the ADC reference VOLTAGE drops out of the equation so as long as it is stable enough that readings of the reference resistor and RTD are made with essentially the same reference voltage, then it can be inaccurate and/or drift around slowly with temperature and/or time. 

Yes, you could calibrate out the initial inaccuracy of a 5% resistor, but a 5% resistor is highly likely to severely compromise the accuracy of reading even a cheap thin-film RTD due to temperature shift and drift over time and other factors.