It simply means that if you want/need more precise calibration, you should use a calibration solution that is close to typical measurements you perform.

So If you usually measure nutrient solutions that are in the 800-1200 ppm range, you should calibrate with a calibration solution of 1000, but if typically performing measurements of lower concentrations (as in 200-500), you should calibrate the instrument with some corresponding solution.

This is only (logically) about precision in a certain range, you may as well measure tap water when the instrument is calibrated with a 1000 ppm calibration fluid but may expect less precision.

Sweet! Thanks!!