The DOT is the U.S. Department of Transportation. Inside it, FMCSA (Federal Motor Carrier Safety Administration) regulates trucking companies: it registers them, audits them, and can shut them down.

Every company that runs trucks across state lines needs a USDOT number. Think of it as the company's federal ID. Companies that haul other people's freight for pay also need operating authority, often called an MC number.

• DOT = the department. FMCSA = the trucking regulator inside it.
• USDOT number = the company's federal ID.
• MC number = permission to haul freight for pay between states.
• Interstate = crossing state lines. That's who FMCSA regulates.