Accuracy is the largest allowable error that will occur
under specific operating conditions. In other words, it is
an indication of how close the DMM’s displayed
measurement is to the actual value of the signal being
measured. Accuracy for a DMM is usually expressed as
a percent of reading. An accuracy of one percent of
reading means that for a displayed reading of 100 volts,
the actual value of the voltage could be anywhere
between 99 volts and 101 volts.
Specifications may also include a range of digits added
to the basic accuracy specification. This indicates how
many counts the digit to the extreme right of the display
may vary. So the preceding accuracy example might be
stated as ± (1 % + 2). Therefore, for a display reading of
100 volts, the actual voltage would be between 98.8
volts and 101.2 volts.