Blue flag
A blue flag tells a driver that a faster car is about to lap them and they must let it past at the first safe opportunity.
In a race it is shown to cars about to be lapped by the leaders. The convention is that a driver has three blue flags to comply; ignoring them risks a penalty, because a lapped car defending its position distorts a battle it is not part of.
In practice and qualifying the flag serves a different purpose — it warns a driver on a slow lap that someone on a flying lap is approaching, where the cost of not moving is a ruined lap rather than a distorted race.
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