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What is the relationship between the "Count" and the accuracy of U3401A or U3402A Digital Multimeter?

The "Count" affects the accuracy of a multimeter. One "count" is defined as a unit change (change of 1) in the last digit of the meter display.

For example:
If you need to measure a DC voltage: 5V, with the U3401A Digital Multimeter, calculate the accuracy by referring to the U3401A's DC voltage specifications table in the Keysight U3400 Series 4 ½ and 5 ½ Digital Multimeters Data Sheet.

Measurement Range: 5.0000V
Resolution: 100uV

From the datasheet, the accuracy of the 5V DC voltage is “0.02% of reading + 4 count”.

Tolerance range
= 0.02% of reading + 4 x resolution
= 0.02% x 5.0000V + 4 x 0.0001 V
= 0.0014 V

Total accuracy = 5.0000±0.0014 V