I know, it will be documented in the v1.0 tutorial (when I find time).
What it does is this:
You can give the probability a number between 1 and 100. When the program finds more then one camera that can be used to look at a car, it will use this probability number to choose which of the cameras is used. A camera with a high probability will have a higher chance of being picked, a camera with a low probability a low chance. There is no chance that no camera is being picked, so in case there are two cameras with probability=10, each camera will have 50% chance of being picked. However, if one camera has probability=10 and the other probability=50, the first camera will have 16.7% chance, and the second camera 83.3%.
When a camera with probability=100 is found, this will immediatly be chosen (so 99 is the highest setting for a camera that is still picked at random). Use this for pit-area cameras, to force them to be used instead of the camera defined for the s/f area.
Cameras without the probability setting have probablity=50.