While there are no tolerance limits specified for the reference resistor in the data sheet, the tolerance does have an impact on the accuracy of the readings. The data sheet specifies a resistor value range - 350Ω(min) 10kΩ(max) - which is related to the reference voltage source. The tolerance of the resistor will be one of the sources of error that the designer needs to take into consideration when designing with the MAX31865. As indicated in the datasheet "The ADC therefore produces a digital output that is equal to the ratio of the RTD resistance to the reference resistance." The greater the value of the tolerance, the greater ADC output error due the error of the reference resistor's actual value. An smaller tolerance value of the resistor means a smaller error. Similarly, the temperature coefficient (TC) will come into play when concerned about error in the ADC result with respect to temperature fluctuations. Therefore using a resistor with a low TC is best.