Coupling factor sets the sampled level, directivity limits separation of forward and reverse waves, and return loss determines how strongly the coupler and its interfaces disturb or re-reflect the signal. Detector accuracy alone cannot define the accuracy of a forward/reflected-power measurement.
Finite directivity leaks part of the much larger forward wave into the reverse channel. That leakage adds vectorially to the true reflected wave, so the error changes with frequency, cable length and mismatch phase. Coupling must also place both samples inside the detector's calibrated dynamic range without compression or noise-floor loss.
Calibrate to the named DUT reference plane, include adapters and cables consistently, terminate unused ports, and verify coupling flatness, directivity, main-line loss and port match over the full band. State the required return-loss or VSWR accuracy and reject any setup whose directivity floor is too close to that target.