What does it mean when multimeter accuracy is marked as: ±0,03%+10Digit?
The 1 digit means that the least significant digit can be off by +/- 1. In this resolution 1 digit would mean +/- 0.001V. 10 digit means that basically of your 79.999V displayed, it could also be 79.989V (not including the 0,03%!)

So basically in your range the 10 digit specification means that +/- 0.03% + 0,01V is your error. For measuring 79.999V it means an absolute maximum error or +/- 79.999*0.03% + 10*0.001V = 0.034V.