My above explanation does not take the potential error of the meter into consideration, and assumes that it is precise at all levels.
To my understanding a precision of 2% over the range of a 0-1999 TDS (ppM) meter would mean that whatever reading you see will be within +/- 2%, so if the meter is indicating 4, the reading might be anything from 3.92 to 4.08.
Of course, this would then have to be multiplied by 2.5, so a reading of 4 ppm/TDS would be a reading of roughly 9.8 ppm silver to roughly 10.2 ppm silver.
Is this understanding all wet? It is how I perceive the statement "accuracy of 2% over the entire range".