What is the difference between the freezing level and 0C isotherm in NOAA's GFS? I'm playing around with NOAA's GFS forecast data and I see that it contains both a "0C isotherm level" and a "Highest tropospheric freezing level".

Could someone tell me the difference between these two levels? I'd assume they are the same except in the case of an inversion.
Various processes affect the freezing point. Both CCN (cloud condensation nuclei) characteristics and drop size affect the freezing point.