Open, Short, Load... OSL calibration.

If you have some kind of VNA you have to be sure your measurements are correct. And this is true: the non-calibrated tool always returns incorrect data regardless of how nice and correctly do they look. To avoid such an inaccuracy of measurements you have to calibrate your device first. There is a well-known procedure named OSL calibration. The OSL calibration is being made against the cable used to measure some device. This is required to subtract the cable from the results of measurements. This is why you have to store results of calibration to separate files (if your device allows you to do this) and then use the corresponding file accordingly.