A DOT number, or Department of Transportation number, is a unique identifier issued by the Federal Motor Carrier Safety Administration (FMCSA) to commercial vehicles and companies operating in the United States. It is primarily used to identify and monitor safety records, compliance information, and incidents related to commercial vehicles to enhance safety on U.S. roads. Key points about a DOT number:

• It is typically a 12-digit or 8-digit alphanumeric code assigned to registered commercial vehicles or companies.
• Required for vehicles that weigh 10,001 pounds or more, transport more than 8 paying passengers, or transport hazardous materials requiring a safety permit.
• The number must be displayed on the vehicle's cab, usually on the driver's door.
• It is used during audits, compliance reviews, inspections, and crash investigations by the FMCSA.
• It helps regulatory authorities track the safety performance of companies operating commercial vehicles and is essential for legal interstate and intrastate commerce involving commercial vehicles.

In summary, a DOT number is a safety and regulatory identification number that commercial motor carriers must obtain to legally operate and ensure compliance with safety regulations in the U.S.