Two things can be inferred from these ratings: the maximum storage temperature is equal to the derated temperature. Also, the slope of the derating can be used to calculate an equivalent thermal resistance in degrees C per watt. The difference between the no-load and maximum full load temperature is less than or equal to the temperature rise at full load. (P-N Designs, Inc.)

Most dc/c manufacturers have design-derating guidelines, but sometimes designers use guidelines they have developed themselves. As engineers, we derate components to improve their reliability by decreasing component stress. (Hearst Business Communications, Inc)

A derating chart is often used, with derating starting at 70oC. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline. (interfacebus.com)