uncertainty estimation for a DMM voltage measurement
If accuracy is ±0.0030% of a 100mV range, ±0.0030% * 100 mV = ±0.00003 * 100 mV= ± 3 μV 

So if your example was  0.1 mV = 100 μV ± 3 μV


Accuracy can be specified differently as a % of reading with a number of counts.  
It can be defined in terms of each part of error; gain, offset, non-linearity
normally examples like yours are expressed in parts per million, ppm

Features             34460A  34461A  34465Anew  34470Anew
Digits of resolution 6½      6½      6½         7½
1 year DCV accuracy  75 ppm  35 ppm  30 ppm     16 ppm

low cost DMM's might be specified in terms of ±% of reading and ±counts