Gauge face wear is the horizontal distance between the measured and unworn profiles at the measured profile's gauge point. Field face wear is the analogous measurement opposite the gauge point.
Using the "floating-point" method of calculating side wear, gauge face wear and field face wear are measured at the gauge point.
"Fixed-point" gauge-face wear is measured at the unworn gauge point, which is defined with reference to the highest point on the unworn rail profile.
Either the "floating-point" or the "fixed-point" method may be used to calculate primary gauge side wear and field side wear. This depends on an options setting when the measurements are calculated. If the "fixed-point" method is chosen, the floating-point method is used to calculate alternate gauge side wear, and vice versa.
More detail on the gauge point calculation is found in the reference section on Modeling Options.