Cars and noncars have bounding shapes, which are used for collisions: They are the collision meshes.
Without bounding shapes, everything would go through each other in the game.
If the bounding shapes aren't exact, objects can sink to each other, or collide too early before the actual impact.
Bounding shapes are done from different shapes. You can have more than one shape, just put 2 (or more) to "Number of 'Bounding shapes' entries".
For cars, there's an easy way to create bounding shapes:
Make a simple mesh to cover the car. (not very detailed: 32 points is maximum)
Then define the triangles, and select "file/export .ASC"
Then start Plaything 2, import your ASC-model, and press SHIFT+B.
Plaything creates a textfile, and just have to copy those bounding shapes into your car's textfile.
Sometimes you can use this for noncars, mostly for simple-shaped noncars, but if your noncar model is a bit more complex, like a lamp post, PT2 can get confused, and spit out huge amount of bounding shapes.
If this happens, you have to divide the ASC-file into simple parts, and make PT2 create the bounding shapes separately.
Then copy the bounding shapes to the textfile, using 2 or more shapes.