Checkline
A checkline in STK is a virtual line preventing unintended shortcuts.
Explanation
Every track has to have a few checklines. They are set so that players have to stay on the road and cannot cut a part of the track. If checklines are set wrongly by track creators or even are missing, exploits are possible. In STK 0.9.3, a lot of tracks included unintended shortcuts because of this.
However, there can also be problems with exisiting checklines. It sometimes happens in particular tracks like Black Forest, XR591 or Hacienda that one misses a checkline because he was going off-road too much. For example, in Hacienda, the checkline at the very start is not long enough, so if you drive too much at the side, you won't validate the checkline and your lap won't count. Because of checkline problems on Ranked, there are sometimes huge ranking changes.