Wheel centre longitudinal offset is a measure of the longitudinal distance between the steer axis and the wheel centre in side view, as shown below.  For steered and driven axles, wheel centre longitudinal offset needs to be kept small to minimise driveshaft plunge and angle.  Typically this should be kept below 10mm for driven and steered axles, although larger values can be used if steering lock angles are reduced or if the driveshafts can handle large angles and plunge.