What changes in the wind
kitcheck.cc uses the forecast's feels-like temperature and then adds a small extra penalty for strong wind, for the airflow of riding into it. In practice it feels a few degrees colder: kit close to a switch point goes a step warmer, and a wind gilet becomes an option when the wind is strong.
With a GPX route, the calculator goes further: it compares the wind direction with your riding direction along the route. A headwind counts fully, a tailwind barely, and the longest headwind stretch shows up in the notes.