National federations are assigned athlete quota spots in the higher levels according to the overall rankings of their individual athletes during the previous season, but are free to send any qualified athlete to a competition in which they have available quota. However, individual athletes' discipline rankings are used to determine the start order for the first heat of each race: the track becomes less smooth after each successive run, so earlier starts are more desirable. In the second (fourth at the World Championships and Winter Olympics) heat, the competitors start in reverse order of their ranking after the previous heat. (At four-heat races, start order in heats two and three is in ascending order of combined time from the prior heats.)