The explanation was about what you wrote. You wrote "0.05%+20mV". If you wanted to say 20 counts, then you should have written "0.05%+20" or "0.05%+20 units" or similar.
If it says 20mV then you add mV, if it says digits, then you add 20 units. It can be either way, mV or digits, depending on how the producer made the specifications.