What matters is the variation in volts. To determine the voltage deviation of a reference specified in percentage accuracy, you multiply the nominal output voltage of the reference by the percentage accuracy and divide by 100. Another reference-accuracy unit you find in data sheets is parts per million. Manufacturers typically use this measure to specify temperature coefficients and other parameters that change little under varying conditions. Converting to a percentage, Overall specifications for a voltage reference list initial accuracy, temperature coefficient, temperature hysteresis, line regulation, load regulation, long-term drift, supply current, ground current, dropout voltage, and load capacitance.
Initial accuracy speaks for itself.
If you take a part off the shelf, connect it in a test circuit, and measure the output voltage, the measured value should be within the initial-accuracy tolerance that the data sheet specifies. This specification usually applies at room temperature only and provides a starting point for most of the other specifications. Temperature coefficient is the deviation of the reference's output voltage from nominal due to a change in the ambient or package temperature. Depending on the device structure and the way the manufacturer trims the output voltage during initial calibration, this output-voltage deviation can be positive, increasing with increasing temperature, or negative, decreasing with increasing temperature. Voltage at the common point, V OUT , is a fraction of the applied voltage, V IN , equal to the ratio of the values of the two resistors.