If I had to explain it, I would say,
What connects to the wire? What is the temperature rating of that connection?
Not all connectors or places where wire lands are rated the same temperature.
Why does temperature matter?
That is the definition of ampacity. The rating at which a conductor can properly conduct without exceeding the temperature of it's insulation. Basically, melting.
So, if see a breaker is rated 40A, then how do you know what temperature they got the rating at? What temperature will the 40A breaker trip at 40A?
That requires some background knowledge on 110.14(C). UL AALZ and their white book, talk about the temperature limitations and ampacities in a given range when the value is not clearly known by product labeling.
What is the ampacity rating of #8 copper thwn-2. that is in free air when landed on a panelboard that says "rated at 60°C" on the inside of the door? 40A. You can not exceed the temperature rating of the board it lands in.
If there is no rating, and it is less than 100A you assume 60°C and 75°C for everything above.
What about the 90°C ratings and the free air ratings? Those can be used to derate the ampacity. Those are never the rating of equipment.
You can, however, have splices rated higher.
A use case for free air would be if you had 3 runs of a conductor connect to the equipment, exit to a cable tray, have a splice and reduce it for the free air temperature rating, then splice again to have 3 runs enter the termination equipment.
Or if you wanted to run 6 current carrying conductors and a load over 100A in one conduit, you could use the 90°C temperature rating to derate but should not be less than the 75°C temperature rating where it lands. For example, one conduit, two runs of 3 phase 208V for a car charger than is 32A load, 40A breaker, and you are running #8 copper thwn-2. You have 6 CCCs, 80% derating, at the 90°C temp, gives you 55A*.8 = 44A which is more than the breaker and load so you are good.
