Drivers may overtake one another for position over the course of the race. If a leader comes across a backmarker (slower car) who has completed fewer laps, the back marker is shown a blue flag telling them that they are obliged to allow the leader to overtake them. The slower car is said to be "lapped" and, once the leader finishes the race, is classified as finishing the race "one lap down". A driver can be lapped numerous times, by any car in front of them. A driver who fails to finish a race, through mechanical problems, accident or any other reason is said to have retired from the race and is "Not Classified" in the results. However, if the driver has completed more than 90% of the race distance, s/he will be classified.