In Digital Multimeter note accuracy in digits
+/- 30 digits indicates the 'absolute error' of the value displayed means that what ever value you read on the display, you have to add / subtract max. 30 from the display value to find the range the actual value is in.

For example your multimeter shows 10.00V then:


add 30 ticks to find the upper limit 10.30V
subtract 30 ticks to find the lower limit 09.70V


So your actual value will be between 9.70V and 10.30V

This also illustrates why you have to choose a measuring range that fits the measured value best. If you choose a too high range, the absolute error will render your measurement completely useless. Look at this example:

Your display reads 00.10V:


add 30 => 0.40V
subtract 30 => -0.20V


Which is an entirely useless result, the actual value ranging from -200mV to +400mV due to bad range and its absolute error. In such a case choose a better (lower) range.



You didn't ask about it, but your relative error was 0.2%. Back to the 9.70V - 10.30V example:


The lower limit will be 9.70V - 0.2% = 9.68V
The higher limit will be 10.30V + 0.2% = 10.32V


So when you read 10.00V on your multimeter, the actual value will range somewhere between 9.68V and 10.32V.