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It’s actually a somewhat important distinction from a medical perspective, although you can figure out the part the matters without actually knowing whether it was a sudden onset or not.
Basically, if it’s a sudden onset, then the person probably still has energy to shiver and has a better chance of rewarming through passive heating (i.e, dry them off and give them blankets). If they’ve been out all day and slowly losing heat, they probably don’t have enough energy left to shiver and will most likely require active rewarming (hot packs). The problematic thing is that if you use hot packs you can actually cause someone to stop shivering even though they would warm up faster by shivering.