A USDOT (U.S. Department of Transportation) number is an identification number that every carrier involved in interstate commercial transportation must obtain from the FMCSA. In case the company does not handle interstate cargo, a motor carrier identification obtained at a state level is generally enough.
Through granting USDOT numbers, FMCSA implements compliance with their safety regulations. Using your carriers USDOT number, you can obtain safety information about the company by calling FMCSA or visiting their site.
Since car shipping brokers do not own any trucks, they are not required to hold such a number. However, when they choose the carriers that will perform the actual shipping, they usually verify such information to insure compliance with federal motor carrier regulations.