The answer is simple: a DOT Identification Number Number is required by the federal law. It represents standardized information about the tire like specific characteristics, capacities and construction. It is also the number the government uses to represent safety standard certification. A tire's DOT Identification Number is not a tire’s serial number. The number represents all of the tires created within the same batch during the manufacturing process.
If your tire doesn't have a federally regulated DOT Identification Number, then it might not be suitable for driving in the United States. In other words, only tires that meet U.S. Federal Motor Vehicle Safety Standards can be legally driven in the United States.
If your tires don't have a DOT Identification Number or if you simply feel like it's time to upgrade your current set, shop by vehicle to create a detailed list of suitable options.
Take a look at the Continental Pro Contact Eco Plus. It has been a popular tire for Prius owners and is quite cost effective.