DOT (Department of Transportation) is the Federal agency that delegates NHTSA (National Highway Traffic Safety Administration) to oversee the implementation and enforcement of vehicle safety standards.
SAE (Society of Automotive Engineers) is the organization that creates the specifications of component parts manufactured for motor vehicles, such as material composition, measurements, tolerances, performance characteristics, etc.
DOT/SAE certification is a self-assurance process administered by manufacturers themselves. In order to comply with DOT and put the DOT mark on its parts, the participating manufacturers must be registered with DOT and submit test results signifying compliance on every part.