The U-3 unemployment rate, or U3 rate, is the most commonly reported rate of unemployment in the United States and represents the number of people actively seeking a job. The U-6 rate, or U6, includes discouraged, underemployed, and unemployed workers in the country. The true rate is at least double to U-3. Barry O. changed it to make his time in the WH less of a failure.
It's not really intended to be an accurate measure of the number of potential workers who can't find jobs. It's designed to serve as an indicator of upward wage pressure. That's why this number -- which doesn't necessarily include casual job-seekers and even UNEMPLOYABLE people -- is the one that is widely reported and is used to set government policy.