Medical odds, or "He's got a 2% chance of making it through the day"

Factual Questions
1

An acquaintance of an acquaintance is in the hospital, and is likely not coming out alive. This morning, his sister was called in to say goodbye. The doctors told her he’s got just a 2% chance of making it through the day.

In one sense, it’s pretty clear what that means: he’s very, very probably not going to make it.

On the other hand, I’m wondering if there’s any rhyme or reason behind the percentages assigned to recovery. Is the 2% (or 50%, or 98%) figure assigned based on how many people in similar circumstances recover? Is it based on the specific case history of the patient being talked about? Or is it just shorthand for “he’s not going to make, but I’m hedging my bet because anything’s possible”?

In short, where does the number come from?

2

When my father was ill, his treating physician explained that when he used percentages like that, he meant that out of 100 patients with similar ailments and case histories, we would see only two survive the night.

I am not a doctor, and of course defer to those better informed than I.