In the WFM601M, cable length is determined by comparing energy at several frequencies with the expected roll-off of the selected cable. If you know the cable type, you can tell its approximate length regardless of source signal level. Source signal level is a measurement of energy at mid-band, modified by the effect of a calculation of the length of the selected cable. If you know the signal level at the source (i.e. 800 mV), you can determine the cable type regardless of length by selecting the type that reads 0 dB. Likewise, if you know the cable type, you can tell the source signal level even if you don't know the length.