What "subzero temperature" mean? Is it below zero F or below zero C? Is it different from "freezing"?
When dealing with a weather forecast in the U.S., subzero means below 0°F. It's a convenient way for the weatherman to say, "It's going to be dangerously cold!"
In a similar way, on a warmer day, you might hear a forecaster saying things like, "with lows in the teens" or "with highs in the upper 20s."
However, at a scientific conference, if I heard a researcher say, "The ion-doped material performed better in subzero temperatures," I'd probably assume she was talking about temperatures below 0°C, since scientists often use Celsius temperature scales, and because that's such a key benchmark temperature on the Celsius scale.
It means below zero on whichever scale is in use or generally understood— centigrade, Delisle, or what have you.
It would mean the same as freezing in scales where the freezing point of water is set at zero, such as Celsius or Réaumur, but not in scales where zero is set to something else, as in Leiden or Rømer.
In Fahrenheit, zero has no particular significance, but it is not uncommon to say sub-zero temperatures to emphasize the coldness (0°F is about -17.78°C).
"Subzero" means literally below 0. It's impossible to say in which scale without context. If the scale is not clear from context, the term is vague.
Since, according to Wikipedia, Fahrenheit scale is used only in 5 countries (Bahamas, Belize, Cayman Islands, Palau and United States), I would suspect, if said by someone from that countries, it's Fahrenheit scale.
In any case I would assume it's Celsius scale.
If the text comes from continental Europe, I'd be sure it's about Celsius scale. Most people there aren't aware of Fahrenheit scale at all.