In many transport analysis studies, the predominant mode of travel is private car. So commuter includes comprehensive tools for modelling as many classes of private vehicle as you require (small cars, large cars, vans, trucks, etc). These are modelled as dynamic intelligent agents that can choose the best route to their destination.
The major difference between Commuter and other tools is that vehicles carrying people are modelled as part of the supply side of the transport system; vehicles are carriers of people, and the demand side is defined by a matrix of person-trips.
Dynamic route vehicles can carry people between parking, drop-off or cordon areas, and serve as a means of transporting a person or people for part or all of their trip.
Empty vehicles can be added to the model as a background volume, to produce realistic delays. For example, if you are interested in studying travel delay, or some other statistic, for people with a particular origin-destination pair, you may want to add background vehicles representing all the other origins and destinations, but not study the individual people inside those vehicles. Commuter's two-level demand allows you to do this.