What does the notation "± (0.7%+3)" mean? Specifically, what does "+3" mean?
The "+3" in that context, is the number of least-significant digits (LSD) on that range, which can be in error. This notation is one of a few "industry standard" ways of specifying measurement accuracy.
The accuracy specification you quoted is from the 2V range of whichever meter is in the question. Therefore using that as an example, the accuracy on a reading on the 2V range is between:
(reading +0.7% +3 LSD) and (reading -0.7% -3 LSD)
I found the Etekcity MSR-A600 meter has the same AC voltage specification as you quoted, so I'll use that one to illustrate LSD a little more. That is a 3.5 digit meter i.e. maximum count = 1999. The maximum count value is important for this next calculation.
For a 3.5 digit meter on the 2V range (actual maximum value shown is 1.999V), the LSD value is 1mV (0.001V).
So the accuracy calculation on that 3.5 digit meter's 2V range, is:
reading ±0.7% ±3mV
(where the 3mV is the 3 LSD, on that 2V range, on a 3.5 digit meter)
The value of the LSD will vary on other ranges. For example, on a 3.5 digit meter's 200V range (maximum value shown of 199.9V) the LSD value on that range is 0.1V.
The "percentage of reading" value is the gain accuracy; the "number of LSD" value is the offset accuracy. They relate to different parts of the measurement process and so have different units.
Some more explanations, for future reference and showing more "worked examples" for different meters:
- BK Precision: How to Read Accuracy SpecificationsBK Precision: How to Read Accuracy Specifications
- Tektronix (Keithley): Specs: How Can I Apply an Accuracy Specification on a Data Sheet to My Specific Measurement (PDF download)Tektronix (Keithley): Specs: How Can I Apply an Accuracy Specification on a Data Sheet to My Specific Measurement
- EE.SE: What does it mean when multimeter accuracy is marked as: ±0,03%+10Digit?
