Details from John William’s Shadow Stats.
The seasonally-adjusted SGS Alternate Unemployment Rate reflects current unemployment reporting methodology adjusted for SGS-estimated long-term discouraged workers, who were defined out of official existence in 1994. That estimate is added to the BLS estimate of U-6 unemployment, which includes short-term discouraged workers.
The U-3 unemployment rate is the monthly headline number. The U-6 unemployment rate is the Bureau of Labor Statistics’ (BLS) broadest unemployment measure, including short-term discouraged and other marginally-attached workers as well as those forced to work part-time because they cannot find full-time employment.