The accuracy of the distance to the fault point for
the cable pair. This value should be added to and
deducted from the value of ccdTdrIfResultPairDistToFault
of the same cable pair to form the upper and lower range
of the distance to fault point for the cable pair.
A value of -1 indicates this value is invalid.
The unit of this value is indicated by
ccdTdrIfResultPairLengthUnit of the same cable pair.