A connector, cable, adapter, fixture and PCB launch each transform the measured impedance. Naming the reference plane tells the supplier and test laboratory exactly where return loss, insertion response and power apply. Without it, two measurements can both be correct yet disagree because they include different interconnects.
Complex S-parameters retain magnitude and phase for every declared port. They support de-embedding, plane extension, time-domain inspection and circuit simulation. Scalar VSWR or insertion-loss values cannot reproduce those operations or reveal the same interaction between a network and its source and load.
Document calibration method and standards, reference impedance, connector condition, port state, fixture model, frequency grid, power level, temperature and uncertainty. Keep the native touchstone file with serial and revision traceability.