Sub-zero in the United States means below 0F. Ambient freezing temp isn't really a "cold" benchmark to people in northern climes, but sub zero (-18C) is a milestone that communicates a cold day from an experiential perspective. For instance, if I tell you a place gets below zero for a couple months, you know it's committed to being frozen, and that circumstantial variations like radiant heat from sunlight aren't going to get you back above 32/0F. Incidentally, freezers are set at 0F/-18C. So being in subzero temps is like being in a freezer, whereas being at 32F/0F could be a completely different experience based on radiant heat from sun, warm winds, etc.