DOT stands for the department of transportation. And it is a sign assigned by the national highway traffic safety administration. Still, they don't test the helmet themselves, the companies undertake their random testing and certify their units themselves.
Moreover, the national highway traffic administration asks the companies to give them samples of helmets, and then they conduct their testing for the safety standards. And those companies who misuse the consumers they are heavily fined. In the list of the above best motorcycle helmets, some are certified, and some aren't.