Well I must be more out of it than I thought, because this is the first time in my 59 years that I’ve heard of the unemployment referred to by the terms U3 and U6.
Don't feel bad, most of us normal people don't have the desire nor the time nor the interest to understand the details of employment statistics; any more than they are interested in the minutiae of a slaughterhouse.
Suffice to say that, there are so many different (reasonable) ways define unemployment, and the range varies so widely that numbers are available to satisfy any propagandist, right, left or center.
Here is a best web page I could find that best describes, superficially, the alphabet soup of employment statistics, and we all can laugh together when we realize that most people discussing the numbers have not the slightest clue how they have been "spun" for specific effect.