A USDOT number is the identifier FMCSA assigns when a company registers with the agency. FMCSA describes it as the unique identifier for the companies it regulates, and it is the number to ask for when checking a carrier's registration.
Under FMCSA's rules, a company needs a USDOT number if it operates commercial vehicles in interstate commerce that meet certain thresholds, such as a gross vehicle or combination weight rating of 10,001 pounds or more, or vehicles that carry a set number of passengers. Carriers hauling hazardous materials in amounts that require a safety permit must register even for intrastate work, and many states require USDOT numbers for intrastate commercial operations as well. A multi-car hauler running between states is well above the weight threshold.
A USDOT number alone does not authorize a company to haul other people's vehicles for hire across state lines. That takes interstate operating authority, identified by a docket number such as an MC number. FMCSA's new Motus registration system adds suffixes to USDOT numbers to show entity type and registrations, and the agency has said existing numbers will not change.
For dealers and auctions, asking for a carrier's USDOT number and checking it in FMCSA's public records is a quick due-diligence step. Related terms: FMCSA and motor carrier.