White flag means that there is a car running at low speed ahead of the observation post where white flag is waved. The display of white flag is performed by waving.
In Japanese races, it regard work cars such as ambulances and car recovery cars as low-speed cars and white flag may be waved.
Regulations
Informs that there is a car running at low speed
If there is a car running at a significantly lower speed than the racing speed, white flag is waved at the previous observation post. If the car running at low speed moves in front of the following observation post, white flag waved at the previous observation post is removed and white flag is waved at the following observation post.
Cars running at low speed due to machine trouble, have a speed difference compared to other cars, so white flag is waved.
In case the car is running at a speed that is likely to stop and is considered to be obviously dangerous to the other cars, yellow flag is waved instead of white flag.
White flag is waved at the observation post previous the car running at slow speed.
出典：youtube.com
Work cars are also regard as low-speed cars
Even if an ambulance or a work car for car removing stopped car is running on the course, white flag is waved as a low-speed car.
During the removing of stopped cars, course marshals are often exist on the course, so yellow flag and white flag are often waved together.
White flag is also waved when a work car that removes a stoped car is intervening on the course.
出典：youtube.com
White flag only alerts
Unlike yellow flag, white flag is used to notify the driver that a car running at low speed is ahead. Therefore, it does not restrict the driving of the driver and there is no violation of ignoring white flag.
Circuit safari
In the Circuit Safari held at SUPER GT, buses that spectator ride run on the course. Therefore, the bus is regarded as a low-speed car, and white flag is waved at the observation post behind the bus.
(10 December 2019)
2.5.5.1 e) White flag
This flag should be waved and is used to indicate to the driver that there is a much slower vehicle on the sector of track controlled by that flag point.