I consider it to be whatever the clever people who design this stuff decide, so that my average use of a vehicle in all potential climates and conditions is factored. Why do I care? My driving habits and conditions haven't changed, nor has anyone else. Things might be slightly warmer, colder, steeper or shallower, but they aren't an infinite high grade against the strongest headwind on earth. That's the kind of lazy talk I'd expect to hear from an engineer who doesn't want to solve the problem and instead would rather see me as a customer suffer.