Calibration is the process of adjusting a scales precision using known weights (calibration weights). Calibration should only be performed if your scale is not weighing properly. After calibration, the scale should display the exact weight that is placed on the tray (within a certain tolerance), and also go to Zero when removing any weight. Typically, a scale calibrates at two points: zero and at the end of its span (span calibration). So, for 500g/0.1g, calibration starts with nothing on the scale (to get the zero weight), then the scale will ask for a 500g weight. Other scales might have linear calibration, which calibrates at 3 points (zero, center span, end span). You should check your user manual for calibration instructions.