Why A vector network analyzer measures both magnitude and phase, so its calibration must remove the errors (directivity, source match, frequency response) in the test port and cables. The standard one-port calibration is SOL: short, open, and load. A short gives a reflection coefficient of -1, an open gives +1, and a precision 50 ohm load gives 0, three known points that let the analyzer solve for its error terms at every frequency.
Watch out Swapping the 50 ohm load for a resonant circuit fails because a resonant circuit's reflection coefficient varies with frequency and is not a known reference; the calibration needs a broadband, precisely known termination.
VNA calibration = SOL: Short, Open, Load (50 ohms). Reflection coefficients -1, +1, 0.