Three preheat regimes (50°F / 100–300°F / >300°F)

So there's three levels of preheat. One is, must be greater than 50 degrees Fahrenheit. The second is, you heat the steel from let's say 100 degrees Fahrenheit to 300 degrees Fahrenheit. And the third level is greater than 300 degrees Fahrenheit. What are we doing with each one of these?