The DOT (or USDOT) is a federal Cabinet department of the U.S. government concerned with transportation. It was established in 1966 and began operating on April 1, 1967. It is governed by the United States Secretary of Transportation.
Its mission is: "To serve the United States by ensuring a fast, safe, efficient, accessible, and convenient transportation system that meets our vital national interests and enhances the quality of life of the American people, today and into the future."
To achieve its mission it legislates regulations for CMVs requiring them to register for a USDOT number. Regulations for CMVs include HOS and DVIRs. If a roadside inspections by DOT officers (or State Troopers) or DOT audit uncovers non-compliance this can hurt a fleet's CSA score.
CMV operators often use compliance software to automate their processes to make sure their drivers and vehicles are compliant at all times.